Add  option and methods to enable, disable or toggle mouse interaction

I'm using WaveSurfer in a project of mine, although I needed a way to enable and disable the mouse interacting with the waveform (I need to stop / play my loop on click instead of moving the progress of the track).

So this should do the job in a non-breaking way, I also added methods on the parent WaveSurfer to change the interaction state and added options / methods to the Readme.

If not compliant to coding standards / you see a potential problem with this please let me know and I'll be happy to iterate on it.

Otherwise I'd hugely appreciate an update of the npm module (I'm currently using this with Browserify).

Cheers!