Implement a `length` option to customize the md5 hash length

I like this task because it does only one thing and does it well! All the other ones I've looked at do too much or just plain don't work.

Anyway I added a `length` option to customize the length of the md5 hash (test and docs updated). I modified `custom_options` instead of adding a separate test, hope that's fine. I also left out examples to keep the docs concise, doesn't seem like anyone would get confused.