WIP: implement #= ... =# multiline comments

This fixes #69, implementing `#= ... =#` multiline comments.

~~(Several people had suggested `#= ... =#` instead, but `#{ ... }#` has the advantage of exploiting automatic editor brace matching out of the box.   Pascal forever!)~~ Changed to `#= .. =#` to make accidental multiline comments less likely (see below).

It doesn't interact nicely with the REPL yet, because the REPL doesn't realize that `#{ ...` means that it should wait for more input.  @loladiro, where is the end-of-input heuristic implemented in the REPL?

I haven't documented it yet, for the simple reason that comment syntax is not currently documented anywhere(!) in the Julia manual as far as I can tell, and I couldn't figure out what section it should go into.