feat: Allow the use of urls instead of files

[Nuno David]

mpv users usually open videos from urls. It would be great to also have that feature in ffsubsync <33

[rudnypc]

up

[AethiQs-Markv]

Hi i was wondering if anybody found a solution to reading both the video and subtitle files directly from a web url and get the synched subtitles back into a Python variable. An answer is much appreciated! @smacke