Make ReleaseCmd add a git tag

[Stebalien]

Specifically, I'd tag it with:

git tag -a "v$VERSION" -m "gx release published as $HASH"

To do this, we'll need:

• Access to the hash.
• The ability to run multiple commands (preferably without bash).
• Decide on whether or not we want to sign the tags.
• Changelogs?

[whyrusleeping]

Yeah, I really want this. @frrist proposed this back in december I remember.

[dignifiedquire]

This is even more important now that gomod is a thing, as it strongly relies on git tags for version information.

[anacrolix]

Lack of tags requires workarounds to use go.mod. A working example using Go modules requires some specific git revisions to work around missing tags in recent releases.