Added onHandleUp and onHandleDown functions

The project I'm working on that uses this slider actually refreshes the page when the values are changed and up to this point I've been having to listen to mousedown and mouseup on the body so that it won't reload until the value changes are finalized. To avoid this, I added option functions that are called both when the handle is pressed (I log values in their current state at this time) and when the handle is released (where I check if the value has changed and refresh the page). If you can think of a better solution I'm open to it, but it works like a dream for me.

Thanks for your work on this project, it's saved me a lot of time!