[Ur] urweb-mode.el improvements

[Adam Chlipala]

Timothy Beyer wrote:
> A while ago, Adam requested improvements to the Emacs mode in the thread "[Ur] Calling all Emacs wizards".  Recently I wrote some (unreleased) preliminary improvements to the mode that mostly address two issues that I have with the mode, although the changes to the font coloring are not yet complete.

Awesome!

> First, the font coloring issue, which is not ready for release yet:

This is the coloring performance issue?  Would be great to see it solved!

> Do you want the indentation to follow urweb-indent-level, or to use the existing method? If you prefer the existing method, then I'd suggest a custom boolean variable urweb-follow-indent-level, which would allow for either method to be used.

I've gotten used to the existing style, so I vote for the custom 
variable, and I'd rather get the sml-mode behavior by default.  (I'm one 
of the rare folks whose first Emacs experience was with SML and 
sml-mode, so it's natural to me. :])