feat: add URL flag type with net/url support

[happysnaker]

Fixes #398

Adds a native URL flag type backed by url.URL from the standard library. Users can now define URL flags that automatically validate and parse URL values.

API

// FlagSet methods
fs.URL(name, default, usage) *url.URL
fs.URLP(name, shorthand, default, usage) *url.URL
fs.URLVar(p, name, default, usage)
fs.URLVarP(p, name, shorthand, default, usage)

// Package-level functions
URL(name, default, usage) *url.URL
URLP(name, shorthand, default, usage) *url.URL
URLVar(p, name, default, usage)
URLVarP(p, name, shorthand, default, usage)

Example

var endpoint = pflag.URL(endpoint, url.URL{Scheme: "https", Host: "api.example.com"}, "API endpoint URL")
pflag.Parse()
fmt.Println(endpoint.String()) // https://api.example.com

[happysnaker]

Closing due to Go 1.12 test failure — URL type requires more work for backward compatibility. Will re-approach.