Add support for right to left mode

Add a new 'direction' option that, when set to 'rtl', moves line numbers to the right and right aligns text.

This is what it looks like in Arabic:

![Arabic Editor](https://f.cloud.github.com/assets/412966/21036/4a15210e-497b-11e2-9aed-8ed836fbf5c0.png)

It still has issues highlighting, navigating through the text, and responding to mouse clicks. I'm not sure how to tackle these issues, but I'm more than happy to take ownership of this use case because it affects me directly.