add method to set what toggleSwitch() display.

[pardo-bsso]

Hi and thanks for your toggleSwitch, it really saved us a lot of time.

We are using it with a custom binding to Knockback (Knockoutjs + backbone), and for that to work we added a couple of things so when the select element behind changes the slider does that too.

Our change is at https://github.com/inaes-tic/mbc-caspa/commit/4da38c9fa002bba34f38271efdeed6417a478589 . Could you please look at it?

Thanks.