Customize what it says in the "results" field

Three changes let users customize their "results" experiences:

1. Allow adding a `Results` worksheet to the spreadsheet (or a second parameter, when using JSON). This lets journalists customize their answers based on how many results are correct. They may use lots of copy/paste, or they may work extra-hard ... their choice.
2. Add a `not_finished_html` option. When `''`, there is no message until the quiz is completed. When `'Keep answering questions...'`, then that's what shows up.
3. Add a `cheating` option. That makes the result message depend upon the user's _last_ answers, not the user's _first_ answers.

They're all in separate commits. I happened to need all three for the project I'm working on.

This depends on PR #15. I couldn't edit until I submitted it.