Disabled dual list

[juanjardim]

Hi, Thank you for creating this library, it's really helpful. I was wondering how can disable a dual list ? after:

$(".dual-list").bootstrapDualListbox();

[jeffersoil]

Pull request created

[russellhoff]

Really useful. Sometimes, you want to make select inputs disabled.

[jeffersoil]

@juanjardim, @russellhoff, Please check https://github.com/istvan-ujjmeszaros/bootstrap-duallistbox/pull/143