Allow options buttons on both sides

Today you can only swipe left to show hidden buttons. This pull request enables options to appear when you swipe right (on the left side) by setting `side="left"` on the `<ion-option-button>` element. It's possible to have buttons on both sides simultaneously, or just on one of the sides (probably a better idea).

This is my first pull request, so please let me know if there's additional stuff I have to do before it is merged in.