Redcarpet and Pygments

### Feature

I wanted to use Middleman to display code samples when used with the `middleman-blog`. This pull-request adds:

* Redcarpet will handle code fences
* Code fences support colorization based on language through pygments

This works if the project adds the following gems:

```ruby
gem "redcarpet"
gem "pygments.rb"
```

And then you specify in the configuration to use `redcarpet` as the renderer.

```ruby
set :markdown_engine, :redcarpet
```

### Discussion

This is not how it should be done, but how it can be done. I wanted to open up the pull-request to discuss if this was a desired feature and the best way to proceed with integration.

* Should the user specify a different markdown_engine?
* Should the user specify redcarpet but then have an additional way to specify pygments?