Allow admins to sudo as any user, not just root

Adds a RunAs spec to the per-user sudoer config.

This makes `sudo -iu metacpan` possible instead of necessitating going
through root first with `sudo su - metacpan`.

I do wonder why each user gets a sudoers file instead of simply adding
the user to the sudo group.
Merge pull request #21 from CPAN-API/sudo-runas

Allow admins to sudo as any user, not just root