Log Message:
Update net/youtube-dl to 2015.12.18.

Update during the freeze approved by jperkin@
(while strictly speaking net/youtube-dl is a leaf package there are various
possible consumers, e.g. multimedia/mpv)

Changes:
2015.12.18:
 o Misc bugfixes and improvements (most user visible change is the fixes
   for #7900 and #7901 that fixes extraction of various youtube videos)

2015.12.13
 o New [funimation] extractor
 o Misc bugfixes and improvements

2015.12.10:
 o Misc bugfixes and improvements