New pull request. Add drain_key_if and remove_key_if to mashmap.

cc @llogiq

https://github.com/imartayan/mashmap/pull/2#event-18297386845

2/ Merged, published, and credited! What!? All in a day. Dang. Hats off to @imartayan.
@shanecelis Thanks Shane, I'm always happy to take contributions and you've made this one very easy to review!