master > Add an option for not adding vex-open class to body when opening a Vex model.

I would like to be able to prevent adding the `vex-open` class to the body element on open. Currently, the only way to achieve this is to manually remove the class with jQuery on `afterClose` and `afterOpen` which is a bit hacky. This PR adds a Vex option called `dontPreventScroll` which, if true, prevents injecting that body class.

Thanks!