Automatically add the "bugfix" label if issue references are detected

[wilzbach]

Should be pretty trivial and could help a bit to filter for pending fixes prior the merge window.

It might be more complicated to automatically remove the label and I guess this won't happen often, so I would go with the simplest case for now.

[MartinNowak]

Sounds good, we have bugfix for phobos, and regression for dmd. Could use the same labels across all repos.

[wilzbach]

Sounds good

See #39 for the implementation ;-)

we have bugfix for phobos, and regression for dmd. Could use the same labels across all repos.

+1 - I don't mind which one you pick.