Add CLI and re-enable cluster

I find myself fiddling with silly bits, like refactoring unreleased code incessantly. I need to start documenting and shipping instead of tweaking and procrastinating. Code review would be nice, if one has spare cycles.

- [x] Add `-h/--help` usage.
- [x] Document CLI.
- [x] Document `npm` config options.
- [x] Document `cluster`-related capabilities.

The level of test coverage (which is now run & checked during Travis builds) in the cluster code leaves something to be desired, but it's tricky to test within the Mocha/Istanbul framework. In any case, the core cluster & CLI patterns have been in production at Zillow for over a year. If I keep noodling around with it in pursuit of nice green numbers, I may never ship these improvements.

Once I knock off the todo list, I'll be merging this in and publishing a minor release (`0.3.0`). There are no back-compat breaks that I am aware of, but a significant chunk of code such as this justifies more intentional upgrade decisions by consumers.