Trimming the https and mp4 check on `:video` tag creation

[cedricsam]

It would be nice if the check could be more permissive:

• allow other protocols, relative links;
• and different file extensions (.webm or other)

I'm already trying a workflow where the .mp4 could be swapped in front-end JS... Could do more find and replace, but thought it might be nice to change at the source!

See suggestion here #179

Maybe it could accept https?://|/ and (mp4|webm|mov|mkv)? Or accept anything and place the burden on the front-end to make proper verifications/rewriting?

[cedricsam]

I re-did the PR because I had to add another branch locally: #183