basemap-gaode: new plugin

Sample China map to illustrate IITC-CE#68.

Currently we can get road map with slightly different look.
It is not straightforward, as there are not much info in official API [docs](https://lbs.amap.com/api/javascript-api/example/).

I've played with some parameters, and you can see result after uncommenting corresponding lines.
Some of the variants allow language change.

P.S.
Different map themes could be seen here: https://lbs.amap.com/api/javascript-api/example/personalized-map/set-theme-style.
Unfortunately I've failed to figure out if it's possible to implement it in the plugin.

plugins/basemap-gaode.user.js

@@ -0,0 +1,92 @@
+// ==UserScript==
+// @id             iitc-plugin-basemap-gaode
+// @name           IITC plugin: Map layers from AutoNavi / Gaode (高德地图)
+// @category       Map Tiles
+// @version        0.1.0.@@DATETIMEVERSION@@
+// @description    [@@BUILDNAME@@-@@BUILDDATE@@] Map layers from AutoNavi / Gaode (高德地图)
+@@METAINFO@@
+// ==/UserScript==
+
+@@PLUGINSTART@@
+
+// PLUGIN START ////////////////////////////////////////////////////////
+
+function setup () {
+  // sample tile: https://webrd01.is.autonavi.com/appmaptile?style=8&x=13720&y=6693&z=14&lang=zh_cn
+
+  var baseUrl = [
+    'https://wprd0{s}.is.autonavi.com/appmaptile?style={style}&x={x}&y={y}&z={z}',
+    'https://webrd0{s}.is.autonavi.com/appmaptile?style={style}&x={x}&y={y}&z={z}&size=1&scale=1',
+    'https://webst0{s}.is.autonavi.com/appmaptile?style={style}&x={x}&y={y}&z={z}', // same as wprd0
+  ];
+
+  var GaodeLayer = L.TileLayer.extend({
+    options: {
+      subdomains: '1234',
+      minZoom: 3,
+      maxZoom: 20,
+      maxNativeZoom: 18,
+      //detectRetina: true,
+      type: 'roads',
+      attribution: '© AutoNavi',
+      needFixChinaOffset: true // depends on fix-china-map-offset plugin
+    },
+    initialize: function (options) {
+      function expand (field) {
+        return options[field]
+          ? '&' + field + '=' + options[field]
+          : '';
+      }
+      var extra = expand('lang');
+      extra += expand('scl');
+      var url = baseUrl[options.site || 0] + extra;
+      L.TileLayer.prototype.initialize.call(this, url, options);
+    }
+  });
+
+  var trafficUrl = 'https://tm.amap.com/trafficengine/mapabc/traffictile?v=1.0&;t=1&z={z}&y={y}&x={x}&t={time}';
+  var AmapTraffic = GaodeLayer.extend({
+    getTileUrl: function (coords) {
+      this.options.time = new Date().getTime();
+      return L.TileLayer.prototype.getTileUrl.call(this, coords);
+    },
+    initialize: function (options) {
+      L.TileLayer.prototype.initialize.call(this, trafficUrl, options);
+    },
+    minZoom: 6,
+    maxNativeZoom: 17
+  });
+
+  function add (name, layer) {
+    layerChooser.addBaseLayer(layer, name);
+    return layer;
+  }
+
+  var Roads = // en, zh_en
+    add('Gaode Roads [zh]',  new GaodeLayer({ style: 7, maxNativeZoom: 20, lang: 'zh_cn' }));
+
+  //add('Gaode Roads',       new GaodeLayer({ style: 7, maxNativeZoom: 20 }));
+  //add('Gaode Roads 7',     new GaodeLayer({ style: 7, site: 1 }));
+  //add('Gaode Roads 8',     new GaodeLayer({ style: 8, site: 1 }));
+  //add('Gaode Roads 8 [zh]',new GaodeLayer({ style: 8, site: 1, lang: 'zh_cn' }));
+
+  add('Gaode Roads + Traffic', L.layerGroup([
+    Roads,
+    new AmapTraffic({ opacity: 0.75 })
+  ]));
+
+  var Satellite =
+    add('Gaode Satellite', new GaodeLayer({ style: 6, type: 'satellite' }));
+
+  add('Gaode Hybrid', L.layerGroup([
+    Satellite,
+    new GaodeLayer({ style: 8, type: 'roadnet', opacity: 0.75 })
+
+    //new GaodeLayer({ style: 8, type: 'roadnet', opacity: 0.75, lang: 'zh_cn', scl: 2 }), // (512*512 tile, w/o labels)
+    //new GaodeLayer({ style: 8, type: 'labels', opacity: 0.75, lang: 'zh_cn', ltype: 4 }) // (feature mask) here: 2: roads, 4: labels)
+  ]));
+}
+
+// PLUGIN END //////////////////////////////////////////////////////////
+
+@@PLUGINEND@@