Git tag convenience function

This is a request for a pair of complimentary functions

```
keybase git tag vX.Y.Z
```

Create a signed tag using the key id associated with the logged in keybase user. (i.e. `git tag -s vX.Y.Z -u keybaseKeyId`)

```
keybase git verify [vX.Y.Z]
```

Verify proofs of a keybase user that signed a tag as well as the normal `git tag -v`

No repository files or tests are named. Start by locating the existing `keybase git` command entry point and compare the requested behavior with `git tag -s` and `git tag -v`. Done means the tag command signs with the logged-in Keybase user's key and verify checks both Keybase proofs and normal Git signatures.

Written by the indexing model from the issue text.