Added the LPPL text to the README.

If this project gets to the point where it can be released, the most
sensible place to do that is on CTAN.  Thus the choice of LPPL.

I'm open to licensing it under other FOSS licences as well.
This changes the way of modifying Markdown's handling of verbatim to
use hooks rather than explicit token lists.
This adds hooks for commands that are "passed through".  The
motivation for this was for using the various matrix commands in itex
in an align environment since `\\` then has to behave differently in
the two contexts.
More experiments with adding numbers to multiline equations.  The CSS
and Javascript code are now split off into separate files.