Implement `wp media import` command

I needed a quick way to import images into a WP file, so this is what I
came up with. It takes a pathname, a list of file paths, or any system-globbed
file path, and calls `media_handle_sideload` on any files found.

Needs tests, I know - I haven't looked through Behat, but I'll figure out 
how to write some basic tests to make sure that it works. 

Does this seem useful? Any thoughts on the implementation?