URL support for opening notebooks

[fonsp]

It would be great if you could paste a URL in the file box and have Pluto download it for you.

(Thanks @alanedelman for the suggestion)

[fonsp]

Added to v0.11.7!

You can paste any URL that points to a (raw) Pluto notebook file.

You can also paste GitHub URLs - Pluto will automatically add ?raw=true
https://github.com/GlenHertz/Pluto.jl/blob/master/sample/Basic%20mathematics.jl

and Gist URLs: Pluto will ask the GitHub API for the list of files and pick the first:
https://gist.github.com/fonsp/19458edf960a380593266cf06d6d7cf9

You can also paste the link to a specific file within a Gist:
https://gist.github.com/fonsp/19458edf960a380593266cf06d6d7cf9#file-lenses-jl