Add a new command-line flag -e EXPR, which runs ghc in interactive
mode and evaluates EXPR only before exiting.

Also, the lexer now ignores lines beginning with "#!".  This is so
that we can use ghc as a scripting language with Unix-style scripts
beginning with

#! /usr/local/bin/ghc -e main

(well, it's not quite that simple, but I'll leave the details for the
more enterprising hackers).