feat: add standalone capability to parse GitHub URLs

[dwilding]

This PR adds a standalone option --parse-url <url>. Usage:

$ gimmegit --parse-url https://github.com/canonical/operator/tree/2.23-maintenance
{"branch": "2.23-maintenance", "owner": "canonical", "project": "operator", "remote_url": "git@github.com:canonical/operator.git"}

The value of remote_url depends on the --ssh option.

This feature is intended for custom extensions to gimmegit.