This patch adds filtering functionality for the author of an issue when listing issues.

In large repositories like heartwood, rad issue list lists more than one screen size of issues. For boiling down this list (for example to find issue IDs of issues oneself has authored), this patch introduces the --authored and --author arguments that are already known from rad patch list.

This makes the interface of that subcommand symmetric to rad patch list, which is desireable from a UX standpoint as well.