Split _rt_cpan in two regular expressions

This allows finer grain of control (and quite frankly, makes it a bit 
easier to read). 

* Fix #914 by requiring RT for - and : separators (RT-1, RT:1), and no
  spaces
* Fix #914 other half by needing RT for # as well for now. If issue
  tracker is github, we still allow without prefix. RT is default
  tracker, so we cannot make broad assumptions here.