Allow options buttons on both sides of a list item

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.