implemented markdown's code block syntax highlighting

I always use and really thank previm!

I saw your [weblog](http://kannokanno.hatenablog.com/entry/2013/06/19/132042) and I'm looking forward code block syntax highlighting.

Why don't you implement code block syntax highlighting ?
Do you have any reason?

I hope you will be satisfied with this pull request.

Thanks.

---
### modification
replaced marked 0.2.9 to 0.3.2
added highlight.js and github.css
added using above open sources' LICENSE

### images
![sample1](https://cloud.githubusercontent.com/assets/2318465/3635748/7bfe6674-0f88-11e4-9174-76eaab3c3820.png)

![sample2](https://cloud.githubusercontent.com/assets/2318465/3635749/86d3519a-0f88-11e4-9f3d-43095c19405d.png)