[DOCS] Added --linkcheckonly parameter for build_docs.pl

[lcawl]

Relates to #249, in particular:

Check cross-doc links and perform other post-build verification on specific books using the already-built versions of everything else.

This PR adds the ability to skip the time-consuming steps related to fetching and building documentation. For example:

./build_docs.pl --all --linkcheckonly
*** USING lcawley-linkcheckonly BRANCH ***
Checking GitHub username and password
Updating repositories
Skipping fetching repo apm-agent-nodejs.
Skipping fetching repo apm-agent-python.
Skipping fetching repo apm-server.
Skipping fetching repo beats.
Skipping fetching repo cloud.
Skipping fetching repo curator.
Skipping fetching repo elasticsearch.
Skipping fetching repo elasticsearch-hadoop.
Skipping fetching repo elasticsearch-js.
Skipping fetching repo elasticsearch-net.
Skipping fetching repo elasticsearch-php.
Skipping fetching repo guide.
Skipping fetching repo guide-cn.
Skipping fetching repo kibana.
Skipping fetching repo logstash.
Skipping fetching repo logstash-docs.
Skipping fetching repo sense.
Skipping fetching repo stack-docs.
Skipping fetching repo x-pack.
Skipping fetching repo x-pack-elasticsearch.
Skipping fetching repo x-pack-kibana.
Skipping fetching repo x-pack-logstash.
Removing old repo <.temp>
Skipping documentation builds.
Checking links
Checking Kibana links
Branch 5.0
Branch 5.1
Branch 5.2
Branch 5.3
Branch 5.4
Branch 5.5
Branch 5.6
Branch 6.0
Branch 6.x
Branch master
All cross-document links OK

The expectation is that this script would be run after a full build had already been done (otherwise the script cannot find the output files and returns an error like this):

Can't open directory html/en/kibana: No such file or directory at ./build_docs.pl line 251.

The idea is that you could run a build for a specific book, copy the output to the appropriate elastic/docs/html/ folder, then run the build --all --linkcheckonly to quickly check for broken links before you merge your documentation changes.