Allow referencing another day by name to use its lessons

[kolbyjack]

When having multiple days with the same lessons, instead of retying the array, allow the user to reference another day by name:

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