Added the `pandocCompilerWithTransformM` function

Following the [discussion](https://groups.google.com/forum/#!topic/hakyll/XbB20ak441Q) on the mailing list, I've implemented the `pandocCompilerWithTransformM` function, which IMHO might come in handy.

Internal links is just one example. One might want to color, for example, disable syntax highlight based on metadata; or mess with the header levels (something that I wanted to do myself).

If you don't want to clutter code with this, I can wrap this into a tutorial of some sort.

Cheers
-Dan