#22 "set" command now supported, and quotation marks in command are
handled.

A new clas, CommandParser, has been introduced to encapsulate all
parsing activity. This way, I can alternate parsing implementations
between regex, standard tokenizer, or fortune teller.

Also, the set command works. I still need to find a cleaner way to
handle interceptors, because my current implementation makes aliasing an
interceptor ("ls" for "dir", "alias" for "set") difficult.