[racket] color partial lexemes

After continuing this discussion off-list with Jon a little bit, I
realized that a large problem was the poor documentation. I've pushed
an improvement now, starting a new section in the 'extending drracket'
manual that starts a list of keys used with read-language's get-info
result (including the colorer).

Currently, there is just a list of eight, well-known token categories
that get colored and it is not extensible. But it seems to cover the
needs for Racket, Scribble, and Honu, at least for now, so I'll wait
until we have some more concrete uses cases before making it
extensible. (Making this extensible leads to new preferences in the
preferences dialog and extensions to the two color themes in DrRacket,
and so it seems non-trivial to do a good job with an extensible
version).

Robby