Refactor to use NodeGit

[iDVB]

Hey @jacob-meacham, Thanks for merging my PR!

I had another thought, that I may not get around to PR'n for a bit. But it would like be a good idea to refactor out all git CLI commands in favor of using something like NodeGit directly.

This will likely mean that the plugin would then work across OSs.

[jacob-meacham]

@iDVB I'm definitely open to a PR doing this! In fact one of the only TODOs in the codebase is to do just that:

serverless-plugin-git-variables/src/index.js

Line 1 in a6a33f5

// TODO: Consider using nodegit instead