Implement new step algorithm

[ephys]

The current step algorithm has a weird behavior when used with minValue & maxValue values that are not multiples of step. (as reported by https://github.com/davidchin/react-input-range/issues/46)

This PR introduces a new step algorithm which:

• rounds the initial value to the next step.
• starts the steps at minValue.
• threats maxValue as a valid step even if it isn't a multiple of step.

eg. with minValue = 2, maxValue = 14, step = 5 Using the current algorithm the steps are: 5, 10 (and a broken initial step of 0) Using the new algorithm the steps would be: 2, 7, 12, 14

This should be considered a breaking change.

[FreeCaribou]

@davidchin Have you had time to look into this PR yet? We'd love to have this in our app

[williamboman]

This makes so much more sense. I just put spent time trying to figure out what I was doing wrong. Please consider merging this.

[johnrom]

I don't think I agree with "Round the initial value to the next step" by default. I think that it should be placed exactly where it would be given a random number outside of the step, and on first drag be placed in an actual step.

I think it makes sense to add roundInitialStep={true} or similar as an option though, so designs using notches could look nice.