Implement skip status

[grocky]

When working in very large repositories like monorepos, running git status --branch --porcelain can take a very long time. In a repo I'm working in it takes nearly 40s to execute. Even when using watchman to precompute file changes, it takes 1.1-1.3s on average. This change allows user to disable the full git status and only get the current branch and stash info.

This implementation is just a global config change to unblock me and allow me to continue using powerline-gitstatus. However, if we can make this configurable per repo or detect when it should be enabled that would be ideal.

[jcharaoui]

I think it's a nice change but would it be possible to limit the changeset to the actual functionality change, and skip the linting?