Support non-github git refs #10
Description
See: https://devcenter.heroku.com/articles/buildpacks#buildpack-references
Would be nice to support generic git references as buildpack URLs, currently only support github.com references (because the tarball API is very convenient)
Using go-git to checkout a particular version should be not too difficult.
Should be a shallow clone + only checkout the ref required.
Might be nice to support using .netrc again for auth?