[youtube] add support for formats 249, 250, 251(opus audio in dashwebm)

[sdsucks]

F:>youtube-dl -F IRrPJsbps-I
[youtube] Confirming age
[youtube] IRrPJsbps-I: Downloading webpage
[youtube] IRrPJsbps-I: Downloading video info webpage
[youtube] IRrPJsbps-I: Extracting video information
[youtube] IRrPJsbps-I: Downloading DASH manifest
[info] Available formats for IRrPJsbps-I:
format code extension resolution note
139 m4a audio only DASH audio 49k , audio@ 48k (22050Hz), 1.21MiB (worst)
171 webm audio only DASH audio 118k , audio@128k (44100Hz), 2.85MiB
140 m4a audio only DASH audio 129k , audio@128k (44100Hz), 3.21MiB
172 webm audio only DASH audio 166k , audio@256k (44100Hz), 3.92MiB
141 m4a audio only DASH audio 255k , audio@256k (44100Hz), 6.37MiB
278 webm 256x144 DASH video 128k , webm container, VP9, video only, 2.60MiB
160 mp4 256x144 DASH video 111k , video only, 2.74MiB
242 webm 426x240 DASH video 283k , video only, 5.20MiB
133 mp4 426x240 DASH video 249k , video only, 6.15MiB
243 webm 640x360 DASH video 541k , video only, 9.38MiB
134 mp4 640x360 DASH video 638k , video only, 13.17MiB
244 webm 854x480 DASH video 933k , video only, 16.65MiB
135 mp4 854x480 DASH video 1170k , video only, 25.20MiB
247 webm 1280x720 DASH video 1805k , video only, 31.89MiB
136 mp4 1280x720 DASH video 2447k , video only, 50.43MiB
298 mp4 1280x720 DASH video 3326k , h264, 60fps, video only, 80.57MiB
248 webm 1920x1080 DASH video 3321k , video only, 55.47MiB
137 mp4 1920x1080 DASH video 4356k , video only, 90.60MiB
299 mp4 1920x1080 DASH video 5685k , h264, 60fps, video only, 136.61MiB
271 webm 2560x1440 DASH video 18553k , video only, 185.30MiB
264 mp4 2560x1440 DASH video 10740k , video only, 205.08MiB
249 unknown_videounknown
250 unknown_videounknown
251 unknown_videounknown
17 3gp 176x144
36 3gp 320x240
5 flv 400x240
43 webm 640x360
18 mp4 640x360
22 mp4 1280x720 (best)

[phihag]

As usual, thank you very much! Will be fixed in the next release.