Design A Configuration API

[cycomachead]

Related to #29

Mostly, I think it should be as self-contained as possible.

Import snap-autograder.js and then that's all you'd need.

let autograder = new Autograder(config)

autograder.runAllTests()

config = {
    callbacks: {}, // TODO
    previousSubmission: (??, ??) => { return snap_xml },
    bestSubmission: (??, ??) => { return snap_xml },
    logSubmission: (submissionLog, serializedXML) { return result? }
}