refactored diff CLI and associated tests

* fixes #2103 (--destination flag was not honoured)

* refactor tests for CLI diff: getting rid of the test dependency github.com/corbym/gocrest
* introduced positional arg parsing with auto-generator doc (see https://pkg.go.dev/github.com/jessevdk/go-flags?tab=doc#Arg)
* refactor output handling: only top-level is controlling the output, all other funcs return io.Reader (easier to test)
* more test coverage (diff pkg is lagging behind the standard expected coverage, which should be above 80%)
* un-export some of the internal "DSL" stuff (however, kept status codes and diff structures exposed)
* updated usage doc

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>

Author: fredbi

diff/fixtures/enum.diff.breaking.txt

@@ -0,0 +1,8 @@
+            BREAKING CHANGES:
+            =================
+            /a/:get -> 200 Response - array[A1].personality : string - Added possible enumeration(s) <sane>
+            /a/:get Request - Query.personality - Deleted possible enumeration(s) <saucy>
+            /a/{id}:get -> 200 Response - array[A1].personality : string - Added possible enumeration(s) <sane>
+            /b/:get -> 200 Response - array[A1].personality : string - Added possible enumeration(s) <sane>
+
+compatibility test FAILED: 4 breaking changes detected

diff/fixtures/enum.diff.txt

@@ -4,11 +4,12 @@ NON-BREAKING CHANGES:
             /a/:get Request - Query.personality - Added possible enumeration(s) <extrovert>
             /a/{id}:get -> 200 Response - array[A1].personality : string - Deleted possible enumeration(s) <crazy>
             /b/:get -> 200 Response - array[A1].personality : string - Deleted possible enumeration(s) <crazy>
-            
+
             BREAKING CHANGES:
             =================
             /a/:get -> 200 Response - array[A1].personality : string - Added possible enumeration(s) <sane>
             /a/:get Request - Query.personality - Deleted possible enumeration(s) <saucy>
             /a/{id}:get -> 200 Response - array[A1].personality : string - Added possible enumeration(s) <sane>
             /b/:get -> 200 Response - array[A1].personality : string - Added possible enumeration(s) <sane>
-            
+
+compatibility test FAILED: 4 breaking changes detected

diff/fixtures/kitchensink.diff.txt

@@ -10,7 +10,7 @@ NON-BREAKING CHANGES:
             /a/:get Request - Query.changeMaxInt - Widened type <Maximum 200.000000->300.000000>
             /a/:get Request - Query.removeMaxInt - Widened type <Exclusive Maximum Removed:true->false>
             /a/:get Request - Query.wideryString - Widened type <integer -> string>
-            
+
             BREAKING CHANGES:
             =================
              Metadata - Spec Metadata - Changed base path </api -> /apibaby>
@@ -24,4 +24,5 @@ NON-BREAKING CHANGES:
             /a/:get Request - Query.changeMinInt - Narrowed type <Minimum 200.000000->300.000000>
             /a/:get Request - Query.changeyPattern - Changed type <Pattern Changed:*->anewpattern>
             /a/:get Request - Query.primToObj : A2 - Changed type <integer -> obj>
-            
+
+compatibility test FAILED: 11 breaking changes detected

diff/fixtures/param.diff.txt

@@ -7,7 +7,7 @@ NON-BREAKING CHANGES:
             /a/{id}:post Request - Header.newboris - Added optional param
             /a/{id}:post Request - Header.optboris - Deleted optional param
             /a/{id}:post Request - Header.reqdboris - Deleted required param
-            
+
             BREAKING CHANGES:
             =================
             /a/:post -> 200 Response - otherDeletedName : string - Deleted property
@@ -18,4 +18,5 @@ NON-BREAKING CHANGES:
             /a/{id}:post -> 200 Response - otherDeletedName : string - Deleted property
             /a/{id}:post Request - Header.changedboris - Narrowed type <string -> integer>
             /b/:post -> 200 Response - otherDeletedName : string - Deleted property
-            
+
+compatibility test FAILED: 8 breaking changes detected

diff/fixtures/path.diff.txt

@@ -3,9 +3,10 @@ NON-BREAKING CHANGES:
             /a/:put Request- Added endpoint
             /a/{id}:post Request- Deleted a deprecated endpoint
             /newpath/:post Request- Added endpoint
-            
+
             BREAKING CHANGES:
             =================
             /a/:post Request- Deleted endpoint
             /b/:post Request- Deleted endpoint
-            
+
+compatibility test FAILED: 2 breaking changes detected

diff/fixtures/response.diff.txt

@@ -5,8 +5,9 @@ NON-BREAKING CHANGES:
             /a/:put -> 200 Response - description : string - Narrowed type <string -> integer.int32>
             /a/{id}:get -> 200 Response - colour : string - Added property
             /a/{id}:get -> 200 Response - description : string - Narrowed type <string -> integer.int32>
-            
+
             BREAKING CHANGES:
             =================
             /a/:get -> 200 Response- Deleted response
-            
+
+compatibility test FAILED: 1 breaking changes detected

diff/fixtures/uber.diff.txt

@@ -5,4 +5,5 @@ NON-BREAKING CHANGES:
             /estimates/price:get Request- Deleted a tag <DeadTagWalking>
             /products:get Request - latitude - Deleted a description
             /products:get Request- Deleted a description
-           
+
+compatibility test OK. No breaking changes identified.