Add FlaskSessionCacheHandler (#833)

Updated examples/app.py
Updated CHANGELOG.md and appropriate docs.

spotipy/cache_handler.py

@@ -2,6 +2,7 @@ __all__ = [
     'CacheHandler',
     'CacheFileHandler',
     'DjangoSessionCacheHandler',
+    'FlaskSessionCacheHandler',
     'MemoryCacheHandler',
     'RedisCacheHandler']
 
@@ -147,6 +148,31 @@ class DjangoSessionCacheHandler(CacheHandler):
             logger.warning("Error saving token to cache: " + str(e))
 
 
+class FlaskSessionCacheHandler(CacheHandler):
+    """
+    A cache handler that stores the token info in the session framework
+    provided by flask.
+    """
+
+    def __init__(self, session):
+        self.session = session
+
+    def get_cached_token(self):
+        token_info = None
+        try:
+            token_info = self.session["token_info"]
+        except KeyError:
+            logger.debug("Token not found in the session")
+
+        return token_info
+
+    def save_token_to_cache(self, token_info):
+        try:
+            self.session["token_info"] = token_info
+        except Exception as e:
+            logger.warning("Error saving token to cache: " + str(e))
+
+
 class RedisCacheHandler(CacheHandler):
     """
     A cache handler that stores the token info in the Redis.