Pass useragent settings to curl

[KOLANICH]

For now useragent is not passed to curl. I guess it'd be logical to pass it.

[dstftw]

> py -3.6 .\youtube_dl\__main__.py BaW_jenozKc -f best --external-downloader curl -v --add-header User-Agent:Foobar
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['BaW_jenozKc', '-f', 'best', '--external-downloader', 'curl', '-v', '--add-header', 'User-Agent:Foobar']
[debug] Adding header from command line option User-Agent:Foobar
[debug] Encodings: locale cp1251, fs utf-8, out utf-8, pref cp1251
[debug] youtube-dl version 2017.02.28
[debug] Git HEAD: 11bb6ad
[debug] Python version 3.6.0 - Windows-10-10.0.10240-SP0
[debug] exe versions: ffmpeg N-83622-gdcd3418, ffprobe N-83622-gdcd3418, rtmpdump 2.4
[debug] Proxy map: {}
[youtube] BaW_jenozKc: Downloading webpage
[youtube] BaW_jenozKc: Downloading video info webpage
[youtube] BaW_jenozKc: Extracting video information
[youtube] BaW_jenozKc: Downloading MPD manifest
[debug] Invoking downloader on 'https://r3---sn-ug5onuxaxjvh-v8cl.googlevideo.com/videoplayback?...'
[download] Destination: youtube-dl test video ''_ä↭𝕐-BaW_jenozKc.mp4
[debug] curl command line: curl --location -o 'youtube-dl test video '"'"''"'"'_ä↭𝕐-BaW_jenozKc.mp4.part' --header 'User-Agent: Foobar' --header 'Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7' --header 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8' --header 'Accept-Encoding: gzip, deflate' --header 'Accept-Language: en-us,en;q=0.5' --continue-at - --verbose --retry 10 -- 'https://r3---sn-ug5onuxaxjvh-v8cl.googlevideo.com/videoplayback?...'

[debug] curl command line: curl --location -o 'youtube-dl test video '"'"''"'"'_ä↭𝕐-BaW_jenozKc.mp4.part' --header 'User-Agent: Foobar' --header 'Accept-Charset: ISO-8859-1,utf-8;q=0.7,;q=0.7' --header 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/*;q=0.8' --header 'Accept-Encoding: gzip, deflate' --header 'Accept-Language: en-us,en;q=0.5' --continue-at - --verbose --retry 10 -- 'https://r3---sn-ug5onuxaxjvh-v8cl.googlevideo.com/videoplayback?...'

--header 'User-Agent: Foobar'