Negative values don't work

[maxp-hover]

If I use a params object like this:

{
  val: -1,
  valMin: -5,
  valMax: 0,
  valStep: 0.1
}

The interface doesn't work properly. The minimum value for the slider works, but not the maximum. The slider goes all the way up to 100 (the default behavior, I believe), not to 0 as I specified.

I might suggest a refactor which internally makes all sliders go from 0 to 1, then implements a remapping function which takes those values and remaps them to the user-specified range. This would solve the issue I mentioned above, but also handle situations where the user provided "min" is greater than the "max". Though not technically correct in a mathematical sense, a user might want bigger values to be on the left side of the slider, and the library might as well support that.