feat(jsonnet): Environment vendor

[sh0rez]

Introduces tkrc.yaml as an alternative project root marker to allow jsonnetfile.json per environment.

Also adds proper docs how to use this new environment local vendor feature.

Fixes #197

[sh0rez]

Also adds a test for making sure rootDir and baseDir are properly found

[slim-bean]

I think we should also update tk init to create tkrc.yml

WDYT @sh0rez ??

[sh0rez]

I think we should also update tk init to create tkrc.yml

WDYT @sh0rez ??

Unsure about this one. jsonnetfile.json works fine as a root marker and I imagine it confusing to introduce tkrc.yaml in new projects, a totally useless file users will wonder about until they require this kind advanced feature.

I'd prefer having this totally non-breaking (and without any noticeable differences), until somebody wants to use this very feature

[slim-bean]

I think I would vote to just have tk init create this file as it will start to ensure it's existing for everyone moving forward and I think in the long run ease confusion. But that being said I don't have a very strong opinion on this, I just want to see it merged as I'm very excited to use it :)