[TxMt] Highlight problem with nested blocks

[Sune Foldager]

Not really sure anything can be done to help it if we want the  
current set of features, but is you type something like this in Ruby:

<<-HTML
<style>
HTML
foo = 21

The 'foo = 21' part and the rest of the document is colored like a  
style-sheet. Normally this would be incorrect nesting of course, but  
for heredocs in languages, there can be good reasons for this (and  
it's in fact used in tmruby).

-- Sune.