Modularize and refactor

[jaxbot]

When this plugin first started out, it made sense to keep it simple. Now that it has grown, the code has become unreadable regardless of how it is arranged. I'll be refactoring it over the next few days.

This will also enable ways to build #15 and #17

[wancharle]

maybe this project help with #15 https://github.com/syngan/vim-gitlab
and thank you for this wonderful plugin.