Image block needs to handle a variety of sources

[cr0ybot]

Currently, the image block by default pulls the first img tag from the description or the contents of a custom tag, but this assumes the tag contents are HTML. There is a setting to use the content itself as the URL, but in the case of podcast feeds, the image URL is provided by an href attribute on the itunes:image tag. Perhaps part of the standard image detection should include checking for itunes:image.

• Check for itunes:image tag as default image (handled in #3)
• Get image URL from custom tag attributes (requires serious rewrite)