add "inheritClasses" option

by default the Tethered element inherit the classes of the element which the Tether is applied to, but this isn't always the wanted behavior., and it's not always easy to remove the original classes.

This could also fit inside the already existing "classes" option, ans could look like so:

classes : {
    inherit : false,
    element : 'myTether'
}

I would do a PR but I don't know how you work and how this whole thing works, and I have my own distinct style and it's really really better if you do it since it's so easy to implement, like 5 minutes :)

Thanks for reading! (fan for years)