Add support for shebangs

[jeaye]

Clojure has reader support for #!. When it sees that pattern, it just ignores the rest of the line. We should update jank to treat #! the same way it treats ;.

This involves updating the lexer to output a comment token when #! is found. We already have the code for handling comments and for detecting #. We just need to add a ! case following # which then reuses the comment code (pull it into a fn named build_comment).

We then need lex tests for the shebang showing up in various places. We can extend the lex comment tests for this.

[bpiel]

I'll take this.