Add support to search multiple markets at once (#526)

* Add support to search multiple markets. Pass in a list or ALL to search all markets.

* pep8 formatting and verification with flake8

* work on comments from stephanebruckert

* pep8 formatting

* run autopep8 formatting

* fix typo

* allow tuple of markets to be passed. Add unit tests for this case

CHANGELOG.md

@@ -7,7 +7,10 @@ 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
+
+ - Support to search multiple markets at once.
+ - Support to search all available Spotify markets.
 
 ## [2.13.0] - 2020-06-25