add @dot markdown

[Dr. Ted Shaneyfelt <ted.shaneyfelt@…>]

Instead of needing to type all of this:

{{{#!dot
  digraph {a -> b}
}}}

markdown that is consistent with Doxygen (and the Doxygen Trac Plugin) should be recognized like this:

@dot
  digraph {a -> b}
@enddot

[Ryan J Ollos]

You'd save a net 6 characters, so the argument isn't hugely compelling. Anyway, the Trac wiki would need to support Markdown to make this happen.