Add optional (opt-in) mixin to create an outline on popovers / tooltips

Created by: brianfeister

Assumes that users will append it to the :after pseudo class of .arrow. Both the size of the outline (perceived thickness of faux outline) and also the color can accept parameters. All math is handled dynamically.

Opting in would look like:

&.top .arrow {
    #popoverArrow > .top( @arrowWidth, @black );
    &:after {
        #popoverArrow > #outlinedArrow > #top > .inner-arrow( @mainArrowWidth, @outlineWidth, @white );
    }
}