Added a mono/stereo toggle

Hi Miroslav,

I stumbled upon your EQ extension and thought it was great, and I had the idea of toggling between mono and stereo, for example if you come across a YouTube video and the sound is only coming out of one ear. So I implemented it into your plugin and as far as I can tell, it works!

All I did was add a 'channelCount' and 'channelCountMode' parameter to the master gain node, and if you switch it between 1 and 2, it'll change between mono and stereo. A simple jsfiddle can be found here: http://jsfiddle.net/Cc6t9/

To be honest, this is my first time sending in a pull request on GitHub, as a matter of fact my first time contributing to someone else's code, so apologies if it doesn't make the cut or if it doesn't follow best practices.

Feel free to contact me about anything. You'd probably want to change the look of the radio buttons - I just used the built-in html elements but you would probably want to make your own icons. Like I said, this is my first time, so I'd expect a few words haha. Thanks!