improve issue links

This is an attempt to improve issue linking in changes and to fix #1045.

It tries to handle links to rt.cpan.org, rt.perl.org, and github issues, and switches between them on a per-link basis.  The github URL is taken from both the bugtracker and repository.  For ambiguous links, it guesses based on the issue number.  Anything below 10000 is assumed to be a github issue and anything above is RT.