Add support for RFC6874

[sgodwincs]

See https://tools.ietf.org/html/rfc6874

[chipsenkbeil]

I just hit this problem where I was expecting this library to support parsing IPv6 addresses as part of a URI, but it failed. @sgodwincs do you have any thoughts on the scope needed to add support for IPv6 addresses?

[sgodwincs]

To be clear, this library does support parsing IPv6 addresses. (unless something is broken that I'm not aware of) This RFC is in particular for supporting IPv6 addresses with zone identifiers. Are you needing to parse IPv6 addresses with zone identifiers?

[chipsenkbeil]

@sgodwincs it turns out that the URI I was trying to parse was malformed and the IPv6 address was not wrapped in square brackets. E.g. scheme://:password@::1:12345 instead of scheme://:password@[::1]:12345.

The confusing part was that I was getting HostError::InvalidIPv4OrRegisteredNameCharacter as the error, which led me to believe that the library did not support IPv6.

[chrysn]

Note to anyone who'd try to fix this: RFC6874 is being revised; the revising draft says that there will be no percent encoding of the percent sign any more. The reviews look positive enough that I wouldn't bother with sticking to 6874 to the letter any more.