Add support for shebangs

[bpiel]

Closes #232

clojure.core=> #! hello
nil
clojure.core=> (+ 1 1) #! addition
2

From the issue:

We just need to add a ! case following # which then reuses the comment code (pull it into a fn named build_comment).

The code I ended up writing for shebang, while heavily inspired by the existing comment code, ended up being different in a few places throughout the code, such that it didn't seem appropriate to me to try to reuse a common function. Let me know if I'm missing something and should change my approach.

[jeaye]

Great work, Bill!