Make default mode from tty to png.

test.py

@@ -17,6 +17,7 @@ class MyWXBot(WXBot):
 def main():
     bot = MyWXBot()
     bot.DEBUG = True
+    bot.conf['qr'] = 'png'
     bot.run()
 
 if __name__ == '__main__':

wxbot.py

@@ -70,7 +70,7 @@ class WXBot:
         qr.border = 1
         qr.add_data(string)
         qr.make(fit=True)
-        if self.conf['qr'] == 'jpg':
+        if self.conf['qr'] == 'png':
             img = qr.make_image()
             img.save(qr_file_path)
         elif self.conf['qr'] == 'tty':