[bitchute] parse timestamp

[markheck]

Checklist

• I'm reporting a site feature request
• I've verified that I'm running youtube-dl version 2020.02.16
• I've searched the bugtracker for similar site feature requests including closed ones

Description

for bitchute, parse the timestamp of a video upload. upload_date is currently parsed, but timestamp is not.

example:

youtube-dl -v -o %(timestamp)s-%(title)s-%(id)s.%(ext)s https://www.bitchute.com/video/mxzcCZI0RQ0k/
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', '-o', '%(timestamp)s-%(title)s-%(id)s.%(ext)s', 'https://www.bitchute.com/video/mxzcCZI0RQ0k/']
[debug] Encodings: locale cp1252, fs utf-8, out utf-8, pref cp1252
[debug] youtube-dl version 2020.02.16
[debug] Python version 3.7.5 (CPython) - Windows-10-10.0.18362-SP0
[debug] exe versions: ffmpeg N-93675-gf857753f56, ffprobe N-93675-gf857753f56
[debug] Proxy map: {}
[BitChute] mxzcCZI0RQ0k: Downloading webpage
[BitChute] mxzcCZI0RQ0k: Checking video URL
[debug] Default format spec: bestvideo+bestaudio/best
[debug] Invoking downloader on 'https://seed132.bitchute.com/i2HYRfKQpdQi/mxzcCZI0RQ0k.mp4'
[download] Destination: NA-Some People are just Glitches in the Matrix-mxzcCZI0RQ0k.mp4
[download] 100% of 27.47MiB in 00:01

the timestamp output parameter in the download Destination is "NA" because it was not parsed. it should be "1582066500" on this particular example (22:55 UTC on February 18th, 2020)