Add .url() parser

[IlyaSemenov]

I think URLs deserve first-class handling in environs, using urllib.parse.urlparse as the parser and throwing the corresponding exception when the value is formatted incorrectly.

I know I can plug it in myself. But URLs are built-in Python functionality and framework-agnostic, I don't see how they are different from, say, timedeltas.

[sloria]

You make a fair case. I would certainly review and merge a PR adding a url parser. =)