Support embed YouTube playlist

[wdv4758h]

This is a tiny modification in shortcode. Let me know if it still need to go through discussion on forum.

This PR adds an extra paramter to YouTube shortcode for including playlist. One can easily use current id parameter to do the same thing, but it looks more like a hack. Adding a playlist parameter seems to be more clear.

To include playlist with youtube shortcode before PR (more like a hack):

{{ youtube(id="dQw4w9WgXcQ?list=RDdQw4w9WgXcQ") }}

After PR:

{{ youtube(id="dQw4w9WgXcQ", playlist="RDdQw4w9WgXcQ") }}

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[Keats]

Thanks!