Updates to ob-racket mode, code review request

[George Mauer]

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.