Add a filter to the option "theme_mods_{$theme_slug}"

I haven't created an issue for this yet. So feel free to use this pull request or implement it in any other way you want to. Or not. :)

This pull request adds a filter to the option "theme_mods_{$current_theme_slug}" so that users can get a filtered url when they use [get_header_image()](http://codex.wordpress.org/Function_Reference/get_header_image), [get_theme_mod()](http://codex.wordpress.org/Function_Reference/get_theme_mod) *(with parameter "header_image" or "header_image_data")* or [get_theme_mods()](https://codex.wordpress.org/Function_Reference/get_theme_mods).

*Thanks for a great plugin!*