Searches tags for various `data-*` attributes and sets `img` tag `src` attribute appropriately. Falls back to searching `noscript` for `img` tags.
Includes unit tests.
* Only processes images with `src` **and** `title` attributes (others are ignored).
* Processes **all** images in the document (not just the first one).
* Wraps the image and its title attribute in a `figure` tag with the title attribute's contents in a `figcaption` tag.
Updated xkcd rewriter unit test.
Added another xkcd rewriter unit test to check rendering of images without title tags.
