Autodetect file type

[nmattia]

A call like niv add https://foo.bar should infer the type file. In particular, a call like this:

$ niv add https://ftp.mozilla.org/pub/firefox/releases/78.0.2/mac-EME-free/en-GB/Firefox%2078.0.2.dmg

should

• figure out that it's a URL and not a github repo
• grab the first alphanum chars after the last char and lowercase

i.e. add the source firefox with type file