Cache rendered HTML.

[octalmage]

Some larger notes take a few extra milliseconds to load.

While it might not seem like a big deal, it's very noticeable when clicking around.

Anytime markdown is rendered, save the output in an array with the ID. Then we can look up the ID and grab the output instead of re-rendering it.

[octalmage]

There's a few bugs caused by deleting or duplicating notes, need modify the cache for these actions too!