Added a class for when slider is "sliding"

[Akkuma]

If you've used YouTube their volume control "expands/collapses" on hover and to guarantee the collapse doesn't occur until the user let's go a sliding class is required.

[mmcc]

Thank you for the PR! I follow what you're referring to about the YouTube player, but you lost me with the rest of the description. What are we trying to guarantee? Just that no slider will collapse while a user is in the process of manipulating it? Is that an issue in this case sense we don't collapse the main slider unless there's no mouse activity and the volume slider doesn't collapse at all?

[Akkuma]

Basically the following situation occurs when building out a YouTube like volume bar.

1. Volume bar hidden by default
2. Move mouse over hover area
3. Volume bar appears
4. Click on handle
5. Drag handle and in the process move mouse outside of hover area
6. Volume bar disappears again

I've resolved 6 by setting a class called sliding when the handle is mousedown, which forces the bar to remain visible until a mouse up occurs, which removes the class and lets the transition effect to occur.

[heff]

Makes sense to me. And looks good, thanks!