Work on a more minimalistic and opinionated version of the syntax file

[fmoralesc]

I had been thinking of working on a version of the syntax file that would remove a lot of what I consider to be cruft or unnecessary. Recent discussions with @alerque made me think that I should do something about it, and here's the product of a few hours, that I can already use with some documents I'm working on. This is in no way intended to be used more widely (it's really messy already :p), but I'm sharing it in case someone wants to try it out.

Over the years, I've come to think that the syntax file tries to highlight too much (and I'm not the only one, although I am less extreme than others). My experience with acme-colors made me think that in markdown documents, it is important to highlight some structure, but not all. For example, headings are important, and it is important for me at least to keep footnotes in a different color from the main text so I can easily skip over when I read. I don't care as much about making references stand out, and I don't care at all about inlines other than pre-formatted text and italics. I don't find the highlighting of list item bullets useful. I can see that I'm dealing with a list from the shape of the text. And so on. My use case has always been geared towards the production of pdf documents through LaTeX, so currently it doesn't provide any of the usual HTML utilities.