Add code block syntax highlighting

GitHub-Flavored Markdown has a nifty feature where you can specify a
language on a code block and it will be syntax-highlighted accordingly:

```c
#include <iostream>

int main(int argc, char* argv[])
{
	std::cout << "Hello World\n";
}
```

We’d like to have this feature as well. Fortunately, there’s quite a few
javascript-based syntax highlighter libraries around. I, for one, like
[highlight.js][] by Ivan Sagalaev.

[highlight.js]: http://highlightjs.org/

It’s easy to integrate, it includes a lot of language definitions and a
GitHub color theme.

If no language is specified, just pass through the code unhighlighted:

```
$ g++ -o hello hello.c
$ ./hello
Hello World!
```