Header Image URL not Handled

When using Tachyon, a single header image doesn't have its image URL tachyoned.

This appears to be because a single header image is never passed through `image_downsize` - it is called directly from `get_theme_mod` where the URL is saved as text. Ref: https://github.com/WordPress/WordPress/blob/master/wp-includes/theme.php#L1052

It looks like this can be filtered via [`get_header_image_tag`](https://github.com/WordPress/WordPress/blob/master/wp-includes/theme.php#L1129) or, even better via [`theme_mod_header_image`](https://github.com/WordPress/WordPress/blob/master/wp-includes/theme.php#L911).

For randomized headers, we might want to filter `wp_get_attachment_url` as they pass through a different function. This doesn't appear to pass through `image_downsize` but I might be missing something.

Worth noting: the srcset for the header image is correctly techyoned, it's the main header image URL that is incorrect.