Add a filter to the option "theme_mods_{$theme_slug}"
which will filter header image url so users can use
get_header_image(), get_theme_mod('header_image')
or get_theme_mods() for example and get a header 
image with the filtered URL.
Added theme mod filter to background image
Bumped version number, changelog and added thanks to myself :)
tabs => spaces
Added toggle_filters() function to avoid code repetition
Documentation