added inline monospace and colorize source block

I added some highlighting that I currently use in vim. 
I'm not an expert in styling, but the basic idea we try to follow - make all the special characters unobtrusive, and vice versa for the actual content - either leave it the same-looking as surrounding text or make it pop.

I'm also not sure about monospace. I would assume most of the people who use Atom have a preferred monospace font set by default. `font-family: monospace` chooses a system's default monospace font and usually looks ugly:
![monospaced](https://cloud.githubusercontent.com/assets/182853/4882399/ae7348c4-6355-11e4-9e41-56bc1cc2427b.png)

What do you think about removing this style block? It will look similar to this:
![default](https://cloud.githubusercontent.com/assets/182853/4882440/5d5b8e6e-6356-11e4-8b93-759b14714d05.png)