Add `--git` option to `mtenv install` to install version by git identifier

[Jens0512]

ATM, mtenv has two ways to install a myst version:

$ mtenv install <github-release tag>

or

$ mtenv install <git commit SHA>

Where <github-release tag> is e.g. v0.6.1 (myst/releases/tag/v0.6.1), and maybe
<git commit SHA>: 32c2a6896ea2e1897ec519853844e30b58326ee1 (or shortened: 32c2a68) (myst#199).

While this works fine, if you want to test with a nightly myst version, you have to manually find a commit SHA from github to install the version. The convention regarding nightly git versions, is to use the master branch or a nightly tag, neither of which are currently supported by mtenv.

Proposal

Initially proposed in the Discord was (quoting myself) "something à la mtenv install master", to which @faultyserver suggested an excellent improvement:

$ mtenv install --git master

Where --git takes an argument: a git branch, tag or SHA. Rendering github release tags the only way to install myst versions without the --git option, which is good, as you only get a non-release version if you specifically say so.

[faultyserver]

Is this handled by #4 as well?

[Jens0512]

No, I thought it was best to put my foot down, and just release what I had before it added to the chaos...