Use 'foreign' to make older ac accept README.md

Closes #32

See merge request !7

Convert README to MD

See merge request !5

Documents can have different languages at different levels. This is
especially true with Mallard stack files, which could incorporate
some translated documents, as well as some that have not yet been
translated. Using a single global parameter completely breaks this.

We were looking for the type as an attr on the root page element,
which isn't what we have in a stack.

This still doesn't address a stack type that is an implicit first
guide and further topics, but that requires deeper surgery.

This allows us to use extra packing divs in linkdivs

We can compute most colors from a set of base colors by moving colors
closer to the text or background color until they hit a target contrast.
For background colors, we were using a very high contrast with the text
color, because the lighter background colors look nice. But when using
inverse colors (white-on-near-black), that caused the background color
computation to give up from recursion depth and return something far
too dark.

Instead, we now detect if it's an inverse color scheme and adjust the
target contrasts accordingly.

This commit addresses one point in this yelp metabug:
yelp#178

Fixes #29

highlight.js renamed cs->csharp and tex->latex, so I had to do the same.
This means that the highlight.js included with yelp-xsl as of this commit
might not highlight those two languages if you drop it into html built
with yelp-xsl before this commit. That's a thing that could potentially
happen in multiple-document sites with partial rebuilds.