Greater support for different kinds of tasks within a sequence.

Tasks in the task sequence are respected. You'll be able to progress through task sequences once I have a final list of criteria for doing so.

Two new silly tasks: identify_letter and type_greek. Identify letter accepts an argument for which letter. e.g. in the Admin interface, you'll see `identify_letter:alpha`, `identify_letter:beta`. 

I also created some sample code for tasks, which you can easily copy if you want to try creating your own task! (Oh the fun!!!)