Jekyll Sub-commands

`bin/jekyll` has grown to the point where it would be easier to maintain in small chunks, rather than as one big script with many different pieces all jammed together.

These changes accomplish three things:

1. Separate `bin/jekyll` into `bin/jekyll` and `bin/jekyll-import` commands
2. Added code to `bin/jekyll` to support sub-commands
3. Add support for the `jekyll init` sub-command, which creates a scaffold in a specified directory or `Dir.pwd`.

This is an alternative plan to @tombell's "The Big Jekyll Command Cleanup" (#690).

I've always preferred working with sub-commands in this way: each sub-command script has only one piece of the responsibility of making Jekyll go, and only deals with one thing.

This will make adding on generators (e.g., `jekyll generate post lalala`) less painful.