Add a Good CLI

[hawkw]

My current command-line interface (Compiler.scala) works, but is pretty basic. If we want to add compiler features other than parsing & compiling one Decaf source code file, such as linking, strictness, choosing the name of the output file, or a REPL, we're gonna need a better command-line argument parser.

@ArcticLight and I are both really big fans of Docopt, the Pythonic Command-Line Parser, so I figure we'll use the Scala port of Docopt.