Insert screenshot for theme preview

I recently needed this plugin for embedding some code in my keynote. I quickly wanted to see which theme fits my needs the best, so I decided to try each one and copy some example code into a RTF file. I made a screenshot and think it might be helpful to others as well.

The only thing you probably want to adjust is the screenshot path. At the moment it's `https://raw.github.com/LeEnno/SublimeHighlight/master/themes.png`, but after merge it will probably be `https://raw.github.com/n1k0/SublimeHighlight/master/themes.png`.