Escape markdown content

Hey @hillna, can you check this branch out, I think this should fix the persistence issues with video embeds. The problem was caused by the embed code not being escaped when loaded into the client side editor. This was causing all the issues when saving the content.

The embed code will now be escaped in the editor, so I'll work on getting a preview feature in here so people can check their work. You'll need to run `npm install` to load the HTML escaping dependency.

Closes #39