Allow styles to configure a custom renderer

Two new examples show how custom renderers can be used to render text along
paths, and to declutter labels using 3rd party libraries.

examples/street-labels.html

@@ -0,0 +1,20 @@
+---
+layout: example.html
+title: Street Labels
+shortdesc: Render street names with a custom render.
+docs: >
+  Example showing the use of a custom renderer to render text along a path. [Labelgun](https://github.com/Geovation/labelgun) is used to avoid label collisions, and [linelabel](https://github.com/naturalatlas/linelabel) makes sure that labels are placed on suitable street segments. The data is fetched from OSM using the [Overpass API](https://overpass-api.de).
+tags: "vector, label, collision detection, labelgun, linelabel, overpass"
+resources:
+  - https://cdn.polyfill.io/v2/polyfill.min.js?features=Set"
+  - https://unpkg.com/rbush@2.0.1/rbush.min.js
+  - https://unpkg.com/labelgun@0.1.1/lib/labelgun.min.js
+cloak:
+  As1HiMj1PvLPlqc_gtM7AqZfBL8ZL3VrjaS3zIb22Uvb9WKhuJObROC-qUpa81U5: Your Bing Maps Key from http://www.bingmapsportal.com/ here
+---
+<!-- Wrap https://npmjs.com/package/linelabel -->
+<script>var module = {};</script>
+<script src="https://unpkg.com/linelabel@0.1.1"></script>
+<script>var linelabel = module.exports;</script>
+
+<div id="map" class="map"></div>