Support for more "shapes"

[monnier]

I really like Ditaa as a kind of "Markdown for figures" and would love to see it better integrated with Texinfo. I also wonder if it could support further kinds of shapes. E.g.:

        B => C      C => D
        ------------------
A => B       B => D
-------------------
      A => D

(for inference rules and proof trees) where the "--" just get turned into horizontal lines, as Ditaa already does, but properly sized/placed (it'd would also be nice not to turn those => into triangles).

     (multiply-by-seven 3)
      \_______________/ ^
              |         |
           function  argument

(taken from Emacs's Intro to Lisp manual) where Ditaa currently gives a poor/weird result (it treats the * as some kind of space/separator).