[youtube] add support for format 266

[sdsucks]

F:>youtube-dl -v -w -c -i -F 1yrp_ZSzRGk
[debug] System config: []
[debug] User config: []
[debug] Command-line args: ['-v', '-w', '-c', '-i', '-F', '1yrp_ZSzRGk']
[debug] Encodings: locale cp949, fs mbcs, out cp949, pref cp949
[debug] youtube-dl version 2014.10.27
[debug] Python version 2.7.8 - Windows-7-6.1.7601-SP1
[debug] exe versions: ffmpeg N-67250-
[debug] Proxy map: {}
[youtube] Confirming age
[youtube] 1yrp_ZSzRGk: Downloading webpage
[youtube] 1yrp_ZSzRGk: Downloading video info webpage
[youtube] 1yrp_ZSzRGk: Extracting video information
[youtube] 1yrp_ZSzRGk: Downloading DASH manifest
[info] Available formats for 1yrp_ZSzRGk:
format code extension resolution note
139 m4a audio only DASH audio 50k , audio@ 48k (22050Hz), 6.79MiB (worst)
140 m4a audio only DASH audio 130k , audio@128k (44100Hz), 18.06MiB
141 m4a audio only DASH audio 256k , audio@256k (44100Hz), 35.87MiB
160 mp4 256x144 DASH video 113k , video only, 15.00MiB
133 mp4 426x240 DASH video 252k , video only, 34.40MiB
134 mp4 640x360 DASH video 610k , video only, 77.23MiB
135 mp4 854x480 DASH video 1115k , video only, 147.96MiB
136 mp4 1280x720 DASH video 2241k , video only, 298.59MiB
137 mp4 1920x1080 DASH video 4342k , video only, 569.49MiB
264 mp4 2560x1440 DASH video 10191k , video only, 1.33GiB
138 mp4 3840x2160 DASH video 24560k , video only, 3.26GiB
266 unknown_video?x3840 22439k , 2.99GiB
17 3gp 176x144
36 3gp 320x240
5 flv 400x240
43 webm 640x360
18 mp4 640x360
22 mp4 1280x720 (best)