add cli

Note: I failed at merging branches so that's why you get so many unrelated commits. You'll be interested in only the ones made May 26th.

This work is in response to #5. The idea here is that we package mozilla-docs as a node module.

Users will install it in their project to obtain the latest release. They will then install it globally to gain access to the cli which affords `mozdoc build` and `mozdoc serve` commands which are analogous to `wintersmith build` and `wintersmith preview` respectively.