Modularize code

[iafisher]

The application can be broken into the following components:

• Command-line parsing and subcommand dispatch goes into main.rs
• Interface to results files goes into persistent.rs
• Data structure definition and logic of asking questions stays in quiz.rs
• Parsing logic stays in parser.rs

[iafisher]

Could also add an iohelper module that holds some of the I/O functions that multiple modules use, like prompt, confirm, and prettyprint.