Why no Marker#getPopup or Popup#getContent methods?

Just curious behind why these methods aren't exposed. I know we can just do `Marker._popup` or `Popup._content`, but in a few recent projects I've needed them and don't feel great reaching into underscore-prefixed properties (might just be my style).

Anyways — just thought I'd bring it up. Happy to submit a PR for my branch, but I figured if they're not there already then there is a reason.
