Make .start() more friendly by asking phone if not given

Ping #530

telethon/telegram_client.py

@@ -231,7 +231,15 @@ class TelegramClient(TelegramBareClient):
                 'function that returns the code you received by Telegram.'
             )
 
-        if (phone and bot_token) or (not phone and not bot_token):
+        if not phone and not bot_token:
+            value = input('Please enter your phone/bot token: ')
+            phone = utils.parse_phone(phone)
+            if not phone:
+                bot_token = value
+                print("Note: input doesn't look like a phone, "
+                      "using as bot token")
+
+        if phone and bot_token:
             raise ValueError(
                 'You must provide either a phone number or a bot token, '
                 'not both (or neither).'