feat(git): refactored git to be a struct

As i will need more commands related to git, i´ll create a struct which
exports all the methods I need instead of creating functions prefixed by
Git*. There will be a instance of the private struct ```Git``` exported
in the utils package.
feat(vkgrc): update json to enforce plugins name
feat(freeze): create freeze command

created a freeze command as in ```pip freeze``` to be able to generate
a valid .vkgrc file from installed plugins.

closes #3
Merge pull request #4 from pepegar/feature/freeze_command

Freeze command