Document argument for unitTask/simpleTask

[grmble]

Right now, unitTask is basically useless - everything in core with no result but side effects needs a Document.

Giving these functions the Document from TaskContext would make them more useful.

[grmble]

In my experience using Bonsai so far, needing the document is the exception. In almost every case I used simpleTask, I've ignored the document arg.

• simpleTask: no-arg, returns a single Cmd
• unitTask: document argument, can not issue commands, for DOM side effects. Unchanged.
• emittingTask: task context, can emit as often as it wants. Unchanged.