Add hex/base64 formatting option to md5 goodie

The sha and md5 goodies do very similar things, but have different look and behaviour. 

This PR implements the option to format md5 hashes in either hex or base64, similarly to the sha goodie's current behaviour, and adds relevant tests.

I'm still familiarizing myself with the instant answer architechture and I need to brush up on my perl skills, so I'm starting out with just a small change.
Any reason this `my` can't be collapsed with the assignment below?