Allow per-day timeslots

[kolbyjack]

Our local schools let out an hour early on Wednesdays, and so I added the ability for a lessons entry to be a map containing timeslots and lessons keys that will apply for that day only.

An example using this feature:

config: {
  schedule: {
    timeslots: ["8:00", "9:00"],
    lessons: {
      mon: ["First lesson", "Second Lesson"],
      wed: {
        timeslots: ["8:00", "8:30"],
        lessons: ["Short lesson", "Short lesson 2"]
      }
    }
  }
}