Generate inline "ditaa" diagrams in README.md files.

Now README.md.src files can use an extended markdown syntax:

    DIAGRAM: foo
    ... multi-line ascii-art ...

which uses ditaa (http://ditaa.sourceforge.net/) to create a PNG
image from the ascii art and put an inline image link in the
outpt markdown.

The files are output to README.md and should look pretty on github.