Adds scaffolding for child theme (based on Twenty Twelve by default)

Apart from creating an empty underscore theme, it would be cool to be
able to create an empty child theme. I've implemented a very basic
command for this. It created a theme folder and style.css which imports
parent theme's styles
rename `scaffold child_theme` to `scaffold child-theme`
make --parent_theme mandatory
style fixes for scaffold child-theme
Land `wp scaffold child-theme`

closes #339