Add "meta" command(s)

[Xion]

There should be a way to obtain information about & do things to the gisht app itself. Some of those information pieces include:

• the gist hosts we support, including the URL format that each one accepts (gisht hosts or gisht services)
• information about currently cached gists and/or gists matching given pattern (gisht ls; requires a little more thought into how it'd be used but "local-only" should probably NOT be the default)
• gisht clean to purge all downloaded gists (probably with confirmation) from the locla cache, or gists of given host, or a specific gist