Emphasis wrapping of highlighted text

[porglezomp]

This is a feature request based on a feature in MarkdownEditing (which I can't use reliably for a few reasons.)
* and _ should behave like parenthesis and quotes do in other code, when you select some text and type *, it should wrap that text with the *. When there is a second paired * in your text, typing another should simply skip it (like typing a closing quote when one has been provided.)

Basically, match the behavior of Sublime's quotes.