Add src attribute to embed shortcode

[aaroncampbell]

The embed shortcode takes URL as its content, but a/v ones take an attribute — make both for both?

See: ​http://make.wordpress.org/core/2013/05/29/post-formats-ui-is-exiting-core-will-live-as-a-plugin/

[SergeyBiryukov]

Related: #24458

[wonderboymusic]

one caveat here - for video, if you want to add subtitles (which we don't support yet - MediaElement does), the body of the shortcode might be a good place to put it:

<track kind="subtitles" src="subtitles.srt" srclang="en" />

Otherwise, HTML frags will have to be passed to the shortcode as attributes. Granted, one could filter 'wp_mediaelement_fallback', but that sounds super ghetto

[kovshenin]

24456.diff​ adds support for URL as the shortcode content for audio and video callbacks, as well as a src attribute to the embed shortcode callback. The src attribute (if not empty) has a higher priority in all three shortcodes.

[wonderboymusic]

I am going to suggest not doing this - see my above comment about <video>, which can contain <track> in its body

[nacin]

I agree with wonderboymusic, so does MarkJaquith per IRC.

In that case, maybe we can add 'src' to [embed].

Punting out of 3.6 as it's not a necessity.

[aaroncampbell]

24456.2.diff​ is just an update to the patch by kovshenin with only the src attribute for embed part

[wonderboymusic]

24456.3.diff​ is a refresh and only sets the URL from attribute if the body (where the URL typically resides) is empty.

[wonderboymusic]

In 28559:

Read the src attribute for the [embed] shortcode if the shortcode's body is empty.

These work:
[embed src="https://www.youtube.com/watch?v=ZqD-LPOWIT0"]
[embed src="https://www.youtube.com/watch?v=ZqD-LPOWIT0"][/embed]

If you do:
[embed src="https://www.youtube.com/watch?v=ZqD-LPOWIT0"]http://any-other-url.com[/embed]

... "​http://any-other-url.com" will be the URL that is parsed.

Props kovshenin, aaroncampbell for making initial patches.
Fixes #24456.