Passing class name to PowerSelect__Options

[Aseem Gupta]

As in title. Since PowerSelect__Options is a tether, it's difficult to access it in case specific class is to be added. The same class which is attached to PowerSelect can be added to that as well.

So, to customize style, we can .PowerSelect.customStyle{...} and PowerSelect__Options.customStyle{...}

[Selva]

${className}__Menu class gets added to the DropdownMenu, where className being the class name passed to PowerSelect.

Added in #16