add new function to `get_youtube_video_id`, eg: `{{ youtube_url|get_youtube_video_id }}`
Valid:
'http://youtu.be/_lOT2p_FCvA',
'www.youtube.com/watch?v=_lOT2p_FCvA&feature=feedu',
'http://www.youtube.com/embed/_lOT2p_FCvA',
'http://www.youtube.com/v/_lOT2p_FCvA?version=3&hl=en_US',
'https://www.youtube.com/watch?v=rTHlyTphWP0&index=6&list=PLjeDyYvG6-40qawYNR4juzvSOg-ezZ2a6',
'youtube.com/watch?v=_lOT2p_FCvA',
'https://www.youtube.com/watch?v=S6q41Rfltsk'
Invalid:
'youtu.be/watch?v=_lOT2p_FCvA'