feature: add configurable highlight to measure line for visibility

[Grammostola]

Seeks to fix #2023

It introduces a highlight to the measure line in order to increase its visibility on various backgrounds, by default a light blue highlight (it was an idea pitched in a meeting some time ago and it is a shorter route than adapting measure to employ the stylewindow that draw uses)

Configurable via a new prop for the measure control:

    {
      "name": "measure",
      "options": {
        "hideWhenEmbedded": true,
        "showSegmentLengths": true,
        "highlightColor": "rgba(230, 126, 34, 0.7)"
      }
    },

[asemoller]

Nice @Grammostola!
Works as expected!

[Grammostola]

Cool!

If no one has any objections I shall then merge this shortly and arrange the necessary documentation.

[johnnyblasta]

I'm getting a error when using the buffer meaure tool:

TypeError: style.getImage is not a function
    at renderFeature (vector.js:122:28)
    at CanvasVectorLayerRenderer.renderFeature (VectorLayer.js:773:68)
    at render (VectorLayer.js:695:30)
    at CanvasVectorLayerRenderer.prepareFrame (VectorLayer.js:715:7)
    at VectorLayer.render (Layer.js:383:23)
    at CompositeMapRenderer.renderFrame (Composite.js:145:29)
    at Map.renderFrame_ (Map.js:1598:20)
    at Map.animationDelay_ (Map.js:1452:10)

[Grammostola]

Thanks for your report! Please try now.

[johnnyblasta]

LGTM