Making markdown engine configurable per slideshow

[grundprinzip]

Sometimes I need to put math on my slides and this was pretty much a showstopper for showoff for me, but then I discovered Marukus math mode (via Jekyll). Since the markdown engine is now selected via tilt, I added a feature that allows choosing the markdown engine per slideshow (with redcarpet as default).

As a result you can write something like this in your showoff.json

    { 
        /*other config*/
        "markdown" : "maruku",
        "maruku" : {
            "use_tex" : true
        }
    }

And this will render the math formulas using blahtex as images and put them on your slides.

Documentation + Tests included.

[goncalossilva]

Out of curiosity, what happens if I omit "use_text": true?

The documentation makes it seem like that option is mandatory. If it is, I think we should specify it by default.

[grundprinzip]

if you omit use_tex it will only use the default of maruku without the special latex features.

[grundprinzip]

To avoid trouble, the settings will per default load the value with false.