Support for pretty="none"

[elliottslaughter]

This adds support for setting pretty="none" to work around #162. Doing so disables the prettifier so that it doesn't mess up the parsing of other Markdown constructs.

The defval = defval or false part was deleted because it was preventing the pretty value from being read from config.ld. Actually, this was a bug even before adding the none value: e.g. you couldn't set pretty="lxsh" before this patch. However, it may be worth double checking if this breaks anything else, since I'm not sufficiently aware of the rest of the code base to know what else this may have interacted with.

[alerque]

Thanks for taking the time to contribute. This looks like a good thing to fix, but I'm not sure this implementation is quite right. Are you still using this?

[alerque]

This line was to cast possible nil values to a boolean false. I think it was probably correct.

[alerque]

See also #118.

[elliottslaughter]

To be honest, I've entirely swapped this out of memory, so I can't remember why the defvar line was removed (edit: aside from what I wrote in the original PR). But yes, we are still using this and the ability to set none (or however you want to spell it, false or whatever) does matter to us.

[alerque]

Can you test your usage with that line put back in? It looks right to me, and there is a big difference between nil and false. It make well be needed, but I'd like to understand why it was removed and what the case(es) are that are affected before I consider merging. Thanks!