Added option to toggle autorendering

First off, awesome work! It looks great and I'm glad someone has picked up the development of a similar project such as Mou.

One thing I noticed from the feedback on hacker news was that the app can use up a ton of processing power when rendering large documents as a result of it happening upon each change. I thought it would be a good idea to have an option (enabled by default) for auto rendering. This way, those working on large documents could turn off the feature and render on command. I provided a keyboard shortcut for this (cmd+R) and it can be found in the View menu.