CPANcover link

I thought it might be nice to have a link to CPANcover to show the code coverage for a module or file.  I've implemented the minimal front-end.  If people think the idea has merit, then there really needs to be cpan-api support to check whether CPANcover has results for the module and, ideally, to link modules to their results, wherever such results may be found (usually lib/ or blib/).

That process seemed a little more complicated (or, at least, I couldn't immediately see how to implement it) so I thought I'd solicit feedback on the general idea first.

The README commit is separate and can be applied (or not) independently.