feat: use cobra to provide subcommands, move sources to lib (filebrowser#506)

- Use cobra in order to provide subcommands `serve` and `db`.
  - Subdir `cmd` is removed.
  - Subdir `cli` is created, which is a standard cobra structure.
- Sources related to the core are moved to subdir `lib`.
- filebrowser#497 and filebrowser#504 are merged.
- Deprecated flags are added. See filebrowser#497 (comment).
- [`viper.BindPFlags`](https://godoc.org/github.com/spf13/viper#BindPFlags) is used in order to reduce the verbosity in `serve.go`.

Author: 1138-4EB

.travis.yml

@@ -17,8 +17,7 @@ stages:
   - release
 cache:
   directories:
-    - vendor
-    - rice-box.go
+    - lib/rice-box.go
 jobs:
   include:
   - stage: lint

Docker.json

@@ -2,9 +2,11 @@
   "port": 80,
   "address": "",
   "database": "/database.db",
-  "scope": "/srv",
-  "allowCommands": true,
-  "allowEdit": true,
-  "allowNew": true,
-  "commands": []
+  "defaults": {
+    "scope": "/srv",
+    "allowCommands": true,
+    "allowEdit": true,
+    "allowNew": true,
+    "commands": []
+  }
 }