Command line argument to clean generated files during `jekyll serve`

A command line argument to clean up the destination directory after `jekyll serve` seems like it would be super handy if you just want to make some quick adjustments to your content, run a `jekyll serve` and take a quick peek, and then commit your changes and leave the working directory clean.

- Added `--clean` command line argument (doesn't change the default behavior of the `jekyll serve` command)
- Updated `kill -9` suggestion to `kill -2` when `--detach` is specified so that the server can shut down cleanly