Use FastImage to detect images that can't be downloaded

[ishields]

I've noticed that meta_inspector returns urls of images that can't be downloaded. When download_images is enabled, we can actually use FastImage to detect non-found images.

This ticket is to track several changes:

1. Add new option to download_image_options hash called "reject_invalid_images"
2. When "download_images" and "reject_invalid_images" options are enabled, we should return nil when the og:image or twitter:image can't be downloaded
3. When "download_images" and "fetch_all_image_meta" option is enabled, return a boolean "valid" indicating whether the image was found to be downloadable.
4. When new option is specified "download_images" and "reject_invalid_images"remove any invalid images since they will not be usable.

[ishields]

(I updated the description of this ticket to cover all features related to validity of the images.)