Added shortcodes 2.5.0 as dependency; Earlier versions corrupted
non-ASCII characters, see
https://github.com/dmulholland/shortcodes/issues/6
The rendered elements have a `shortcode` CSS class.
The YouTube shortcode supports various ways to refer to a video:
- `{youtube VideoID}`
- `{youtube youtube.com or youtu.be URL}`
URLs containing an '=' should be quoted, or otherwise the shortcodes
library will parse it as "key=value" pair.
The IFrame shortcode supports the `cap` and `nocap` attributes. `cap`
indicates the required capability the user should have in order to
render the tag. If `nocap` is given, its contents are shown as a message
to users who do not have this tag; without it, the iframe is silently
hidden.
`{iframe src='https://source' cap='subscriber' nocap='Subscribe to view'}`
Merged test code + added HTML class for shortcode iframes
