Polishing, mostly documentation updates.
Updating plugin docs.
Enabling to specific a set of plugins with the -N option.

Bro will then print information only about the plugins specified.
Adding a test building a file analyzer plugin.

Also cleaning up, and moving, the other plugin tests.
Updating plugin hook test.
Adding missing method implementation.
Merge branch 'topic/robin/dynamic-plugins-2.3' of git.bro.org:bro into topic/robin/dynamic-plugins-2.3