Ability to extract media from entries

Expected Behavior

Add an API, entry.media, which renders the HTML of an entry and extracts out the elements which could be considered media attachments in a feed context - img audio video.

Possible solution

Provide the attachments as a list of property bags, e.g.

[{
    'href':'http://example.com/foo/image123.jpg',
    'content-type':'image/jpeg',
    'attachment-type':'photo'
 },{
    'href':'http://example.com/bar/video.mp4',
    'content-type':'video/mpeg-4',
    'attachment-type':'video'
}]

Semantic-type should be compatible with mf2 u-type specifiers (e.g. u-photo). This way we can support both Atom/RSS and mf2 feeds.

The attachments function should be a CallableProxy that accepts the standard rendition args (max_width/max_height etc.).

Possibly support srcset although it's unclear if that would provide any real benefit in this context.

Context

Provides better attachment support for rich feed readers that understand attachments.

Relevant IndieWeb Chat discussion

PlaidWeb / Publ