Accept usage of Flask Blueprints as app instance

Author: maryum375

slackeventsapi/server.py

@@ -1,4 +1,4 @@
-from flask import Flask, request, make_response
+from flask import Flask, request, make_response, Blueprint
 import json
 import platform
 import sys
@@ -18,7 +18,7 @@ def __init__(self, signing_secret, endpoint, emitter, server):
         # If a server is passed in, bind the event handler routes to it,
         # otherwise create a new Flask instance.
         if server:
-            if isinstance(server, Flask):
+            if isinstance(server, Flask) or isinstance(server, Blueprint):
                 self.bind_route(server)
             else:
                 raise TypeError("Server must be an instance of Flask")