Allow open_browser to be set on creation of auth_manager (SpotifyOAuth/SpotifyPKCE) (#574)

* Fix plamere/spotipy#560

* Adhere to code style

* Update CHANGELOG.md for changes in 939f869bbc

* open_browser passed to get_auth_response should take precedence over the same argument in the constructor

* Update FAQ

* Add information about headless auth

* Add headless auth example

CHANGELOG.md

@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
-// Add your changes here and then delete this line
+### Added
+
+- `open_browser` can be passed to the constructors of `SpotifyOAuth` and `SpotifyPKCE` to make it easier to authorize in browserless environments
 
 ## [2.15.0] - 2020-09-08