allow the command format `git pr checkout pr/#`

it's a little silly, but because the branches get named that way,
sometimes I try to check out the branches with this format.
filter non-digit from pr arg & use curly braces
moved pr number parsing before error check
don't try to parse if a number isn't given
Updated README
Added uncommmented option to fatpack git-pr

Still not satisfied with how it works. I actually want a fatpacked git-pr
to be generated and copied back into the root dir, so that a fatpacked
script can be put on GitHub, but the normal version should be installed
when you use the CPAN toolchain. Hence disabled.
Merge branch 'master' into pr/8
Cleaned up logic in checkout command - Guard clauses FTW