Improvements for "headless" mode

[phated]

An extrapolation on #108, I've made some observations on how this package could help enable publishing from CI:

• Accept GitHub token from environment (ref Accept GitHub Token from the environment? #108)
• Option to disable opening browser upon PR creation. Currently the command crashes if open or xdg-open exists but doesn't succeed.
• Check if git user & email are set so the commit command doesn't fail without a good error message. Optionally accept a user and email at command line to use for commit.
• Allow configurable protocol, or always use https and append token to the command for pushes. e.g. git remote add user https://[email protected]/phated/opam-repository will work because the GitHub Token used for forking will also allow pushes to that repository.

My working GitHub Actions template (with workarounds) is at https://github.com/grain-lang/binaryen.ml/blob/e2fb4dafa5cf90757c6183409ca04edc76d84db1/.github/workflows/opam-publish.yml