Support sprites for admin menu icons in register_post_type and add_menu_page

[helenyhou]

We should encourage developers to maintain a consistent and beautiful admin UI by making it easier to register sprites with black and white and color versions of icons for the admin menu, namely in register_post_type() and add_menu_page() (and its wrappers).

Related: We should also do an education push and create an external web tool to make it easy to generate a properly sized, colored, and positioned sprite.

Related tickets: #20036, #19886

Initial thought: have the menu_icon arg and $icon_url param take an array with a bool for whether or not it's a sprite, defaulting to false.