FR: Allow `box()` to access distant modules through a URL

[DanChaltiel]

Please describe your feature request

Hi,

Thank you for this incredible package!

I mostly use box to share small sets of documented functions when creating packages would be overkill.

In that context, it would be tremendously handy if one could point to a URL. I could use devtools::source_gist(), but box offers the great advantage of documenting its functions.

For instance, I'd love to be able to write something like the following:

url = "https://raw.githubusercontent.com/klmr/box/b386bac4d7269f989f63c1fd650e8be464dd44ed/inst/mod/hello_world.r"
box::use(world = url)

PS: the link to your contributing guidelines is 404 dead.