Find video URLs on a page

[Farzat07]

Just like Video DownloadHelper. This is already in the plugin ideas section in https://github.com/qutebrowser/qutebrowser/issues/30#issue-44554296, but I thought some people might actually check this repo only so decided to open an issue here as well.

Would be really nice if one could perform custom commands on the urls, such as open them in a player such as mpv.