Add support for complex compound assignments where the LHS is a scalar.

Fixes <rdar://problem/11224126> and PR12790.
<rdar://problem/11914077>

If you type help command <word> <word> <word> <missingSubCommand> (e.g. help script import or help type summary fake), you will get help on the deepest matched command word (i.e. script or type summary in the examples)
Also, reworked the logic for commands to produce their help to make it more object-oriented