Add option to create invisible mask layer

The apps that I work on have many controls which perform data modification, and I thought it would be a nice feature to have Intro.js create an invisible layer (with the same dimensions as the `helperLayer`) to disallow interaction with the current target element.

It can be enabled by settings the `createMask` option when calling `.setOptions()`. By default, it is not enabled.

Thanks for the great project!