PATCH: highlighting of heredocs used as method args
This should highlight heredocs used as method args properly. Such as:
puts <<FOOBAR, $foobar
However, I haven't come up with a good solution for this yet:
foobar <<ABC, <<DEF, $foobar
So I've added 'keepend' to the heredoc syntax group definition, until I
can come up with a better solution, which should minimize the chances of
the highlighting running away after the final delimiter.