Add support for customizing target's url

[devxoul]

Summary

This PR provides an way to customize the target's url.

Background

Imagine that we have an API which returns the next url path or url:

{
  "data": [
    {...},
    {...},
    {...},
  ],
  "next": "/articles?page=2"
}

In this case, we can add a case path(String):

enum MyAPI: SugarTargetType {
  case path(String)
  case user(Int)

  var route: Route {
    switch self {
    case .path(let path):
      return .get(path)
    case .user(let id):
      return .get("/user/\(id)")
    }
  }
}

Default implementation of Moya uses URL.appendingPathComponent(_:) to make URL, so the URL will become http://example.com/articles%3Fpage=2 (? becomes %3F)

Customizing URL

With this PR, you'll be able to customize target URLs.

extension MyAPI: SugarTargetType {
  var url: URL {
    switch self {
    case .path:
      return URL(string: self.baseURL + self.path)! // custom url
    default:
      return self.defaultURL // default url
    }
  }
}