Add an option to mute a chat

[powerfrontier]

I'm using this plugin on Pidgin and I really miss the option to mute a chat, like the android app. I've gone through this plugin and Pidgin options and I have not seen any one to accomplish that. I open this issue here because I think it is responsibility of the plugin and not Pidgin, but if it is the opposite tell me and I will close this and open one on the Pidgin side. There is an open issue already #88 , but I'm not sure if it is exactly the same issue there and also it seems outdated.