Optional highlight support

I like to highlight stuff in my blog a lot. I normally just change italic to highlight, but I thought it would be nice to make an explicit highlight over italic. This is my proposal:

    This is ==highlighted==.

I considered doing `__highlighted__` but leave `**bold**`, but I thought the double equal might be nice since it doesn't stomp on any existing Markdown syntax.

I updated the readme and added a passing test. I'd love to see this change incorporated. Let me know if you want me to do anything :smile: