Add Pandoc mode

[madman-bob]

This creates the executable katla-pandoc. When run as a Pandoc filter, this runs idr code blocks through Katla. There is an example md file in the tests, and running the tests generates the PDF.

It's complete enough for my purposes, but there are a couple of niggles:

I originally wrote this separately to Katla, so it calls Katla as an executable. It could be integrated more closely with Katla by using Katla as a library, but I didn't want to dig that deep.

I generate the Katla macros, and then immediatly call them, which is not ideal. I didn't see a way to generate inline code.

There seems to be a small space before inline snippets. It's most obvious in the Types section of the test PDF. I didn't dig into LaTeX enough to figure out why that is.

[andrevidela]

I was going to need this exactly today

[andrevidela]

After trying this out, I'd be in favor of merging this. But I'm not familiar enough with Katla's internals to review this at this stage

[andrevidela]

I've been relying on this long enough that I'll just merge it.

Can it be installed with pack as a binary? I think it should but I haven't looked into it