Add helm-git plugin instead of helm-github (#375)
Add helm-git plugin instead of helm-github (#375)

We would like a a way to release from non-packages helm repositories, 
directly from git, with a ref and path specification. See [1]

`helm-git` supports `git` scheme making it compatible with `helmfile`

This closes [2]


[1] https://github.com/roboll/helmfile/issues/435
[2] https://github.com/cloudposse/geodesic/issues/355
cloudposse/geodesic