[github] allow repos to be sepecified with simple regex expression

[rabbah]

Is your feature request related to a problem? Please describe.
In some cases it is convenient to bind the github commands to a set of repos, expressed as nimbella/* or nimbella/commands-*. Currently each repo would have to be listed explicitly.

Describe the solution you'd like
Allow github_repos to accept a pattern, specified with simple regex (support . and *?) instead of requiring an explicit list.

Additional context
The question does arise as to when to resolve the list. I think the simplest case is to use the PAT/bearer token to list all repos that are accessible, and filter by regex match on every command activation.

Additionally, it would make sense to then also support the same regex expressions on the command line overrides.