New renderTransparent() configuration option

[ahocevar]

This pull request adds an option to render transparent content, e.g. invisible polygon fills, for hit detection. Example: when you want to have parcels rendered only with their outline, but want the whole parcel to be clickable, configure your style with

  "layers": [
    {
      "id": "parcel",
      "source": "parcels",
      "type": "fill",
      "paint": {
        "fill-color": "rgba(0, 0, 0, 0)"
      }
    }, {
      "id": "parcel-outline",
      "source": "parcels",
      "type": "line",
      "paint": {
        "line-color": "rgb(0, 0, 0)",
        "line-width": 0.5
      }
    }
  ]

In your application code, configure the new option:

import { renderTransparent } from 'ol-mapbox-style/dist/stylefunction';
renderTransparent(true);

Fixes #108.