fix: full support for Protocol Version 2 (#13)

BREAKING CHANGE: I ran into issues with the `git-http-backend` npm module when trying to add support for Git Protocol Version 2 support, so I ended up ditching it and modifying my code to replace that functionality. This change doesn't break any isomorphic-git tests, but I'm releasing this as a major version bump just to be cautious.

fix: update CORS to allow Git-Protocol header (#12)

feat: set GIT_PROTOCOL env var from Git-Protocol header (#9)

1.2.1

1.2.0

1.1.0

1.0.0