Rewrote MainModule.readPrompt to require ";;" to run code

This will make parsing blocks much easier, but of course requires that
the user enter ";;" to run whatever code has been entered. At some
point in the future, it may be nice to allow this to be turned off in
interactive prompts, given the understanding that blocks won't work.
Rewrote MainModule.runInteractive

Made the code that checks for "quit" or "exit" a little easier to deal
with, and so a little less lambda-y.
Added Parser.EOL shorthand
Added Parser.parseSubExpressions

Currently matches parseLiteralValues, will need to add method calls and
the ability to match multiple values with infix functions, such as "+",
"-", "/", "*", etc.
Will want to work on the infix functions first.