ClassToggle: Add ability to explicitly add/remove classes

ClassToggle can now explicitly add or remove classnames from target
elements. To do this, simply prepend -- or ++ to explicitly
remove or add class names to the target element(s). For example, to
toggle one class whilst explicitly removing another, use:

data-classtoggle-class="toggle-this,--remove-this"

To remove one class whilst adding another, use:

data-classtoggle-cass="++add-this,--remove-this"