Support Protocol-relative URL (PRL)

[besfahbod]

Levels of support can be like:

1. Parsing PRL strings into an object and then back to the string should work out of the box.
2. Allow generation of PRL from a relative URL.
3. Allow downgrading a URL with protocol into a PRL.

Specifically, (1) is very important when using the library on href values, which can be intentionally kept as PRL, for better client support.

References:

• https://en.wikipedia.org/wiki/URL#prurl

What do you think?

[Mikhus]

Would you like to consider PR for this? :)

[besfahbod]

Will try to find the time for it. (Probably need to fix the path encoding issue, first, though.)