Add bot_token argument (#221)

* Add bot_token argument (closes #123)

* Make session_name docs more readable and detailed

* Explicitely set is_bot=False

pyrogram/client/ext/base_client.py

@@ -68,7 +68,7 @@ class BaseClient:
     }
 
     def __init__(self):
-        self.bot_token = None
+        self.is_bot = None
         self.dc_id = None
         self.auth_key = None
         self.user_id = None