Normalize domain names to lowercase

[millette]

Since domain names are case insensitive, I'd like an option to normalize
"SindreSorhus.com"
into
"http://sindresorhus.com"
while keeping the path and query strings intact.

It would make finding duplicate URLs slightly easier.

[sindresorhus]

I don’t think there’s any reason for an option for this. Lets make it default behavior and do a major release.

[millette]

Even better :-) I can take a stab at a PR if no-one else is.

[millette]

See #99 - although it already works as expected. I only added a test.