Images: Allow a filter to disable image resizing via WPCOM

Allow users to set a `jetpack_images_fit_image_url_override` filter which if
set will be used to determin the url to a resized image. This allows users to
bypass Photon / WordPress.com.

Resolves #1083
Merge pull request #1313 from xyu/fix/issue-1083

Images: Allow a filter to disable image resizing via WPCOM