Updates to ob-racket mode, code review request

[George Mauer <gmauer@gmail.com>]

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I'm a big fan of using org mode to learn languages and have been using it
for Racket (sorry Dr Racket, I just love emacs too much). It's been mostly
fine and I've made a few patches to the [`ob-racket`](
https://github.com/xchrishawk/ob-racket) I've been using. Recently I've
started to move into the build-your-own-language parts of Racket and found
that ob-racket didn't support custom language directives. So [I modified it
to support them](https://github.com/togakangaroo/ob-racket).

I'm pretty new at elisp, so would anyone be willing to give me a code
review to make sure I didn't do anything completely stupid? [It is a single
not-too-large file](
https://github.com/togakangaroo/ob-racket/blob/master/ob-racket.el).

Also looking for opinions on whether this is helpful enough/good enough to
be worth pushing to melpa.

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