Fix multilanguage for bedrock

Add language support with composer pre- / -post hooks, allowing to define WPLANG the normal way, fixes roots/bedrock#30. Compatible with plugins such as WPML.

 I can now set WPLANG to nb_NO and go into the wordpress dashboard and see nb_NO. I click "Update" and I got the new language installed.

Basically symlinks are created on composer install/update, these symlinks are ignored in the .gitignore file so that deploying with capistrano works as well.

Also tested with WPML plugin that it works.