Add gpk tag command

[braydonf]

This could include an additional hash into the signed tag.

Example usage:

• gpk tag <version> - This would create a signed git tag for the version with additional sha512 tree, (e.g. git tag -s <version> -m <version+sha512tree>).
• gpk tag <version> <hash> - Sign a specific commit.
• gpk tag <version> --algo=sha512 - Use a specific hash function.

Example code:

• https://github.com/braydonf/gpk/blob/master/lib/git.js#L186-L210