Add an optional "origin" option to authorization middleware.

This allows user to specify an "origin" URL only to which site the
authentication credentials given are passed via request header.

```
conn = Faraday.new { |faraday|
  faraday.request :url_encoded
  faraday.request :basic_auth, 'user', 'password1', 'http://example.org/private/'
  faraday.request :basic_auth, 'user', 'password2', 'http://example.net/'
}
```