Fix/reenable allow overlap for large sliders (ptomasroos#209)

* fix: Reenable allowOverlap for large sliders

If your slider goes from, let's say, 0 to 10:
- `this.optionsArray = [0, 1, ... 10];` => its length is 11
- however, `this.props.sliderLength` should only be divided by 10 (representing the 10 sections `0-1`, `1-2`, ..., `9-10`)

The fix is then simply to replace L56 by
`    this.stepLength = this.props.sliderLength / (this.optionsArray.length - 1);`

* docs: Explicit the unit used by minMarkerOverlapDistance for more clarity

One could think it was about steps

Author: AmauryLiet

MultiSlider.js

@@ -54,7 +54,7 @@ export default class MultiSlider extends React.Component {
     this.optionsArray =
       this.props.optionsArray ||
       createArray(this.props.min, this.props.max, this.props.step);
-    this.stepLength = this.props.sliderLength / this.optionsArray.length;
+    this.stepLength = this.props.sliderLength / (this.optionsArray.length - 1);
 
     var initialValues = this.props.values.map(value =>
       valueToPosition(

README.md

@@ -99,5 +99,5 @@ Feel free to contribute to this part of the documentation.
 | markerOffsetX | 0 | number | Offset the cursor(s) on the X axis |
 | markerOffsetY | 0 | number | Offset the cursor(s) on the Y axis |
 | markerSize | 0 | number | It determines the marker margin from the edges of the track, useful to avoid the markers to overflow out of the track. |
-| minMarkerOverlapDistance | 0 | number | if this is > 0 and allowOverlap is false, this value will determine the closest two markers can come to each other. This can be used for cases where you have two markers large cursors and you don't want them to overlap. Note that markers will still overlap at the start if starting values are too near. |
+| minMarkerOverlapDistance | 0 | number | if this is > 0 and allowOverlap is false, this value will determine the closest two markers can come to each other (in pixels, not steps). This can be used for cases where you have two markers large cursors and you don't want them to overlap. Note that markers will still overlap at the start if starting values are too near. |
 | imageBackgroundSource | undefined | string | Specifies the source as required by [ImageBackground](https://facebook.github.io/react-native/docs/imagebackground)|