Add a priority option to addOverlay

See https://discuss.codemirror.net/t/addoverlay-removeoverlay-and-opaque/835

doc/manual.html

@@ -1435,11 +1435,21 @@ <h3 id="api_configuration">Configuration methods</h3>
       spec</a> or a mode object (an object with
       a <a href="#token"><code>token</code></a> method).
       The <code>options</code> parameter is optional. If given, it
-      should be an object. Currently, only the <code>opaque</code>
-      option is recognized. This defaults to off, but can be given to
-      allow the overlay styling, when not <code>null</code>, to
-      override the styling of the base mode entirely, instead of the
-      two being applied together.</dd>
+      should be an object, optionally containing the following options:
+        <dl>
+          <dt><code><strong>opaque</strong>: bool</code></dt>
+          <dd>Defaults to off, but can be given to allow the overlay
+          styling, when not <code>null</code>, to override the styling of
+          the base mode entirely, instead of the two being applied
+          together.</dd>
+          <dt><code><strong>priority</strong>: number</code></dt>
+          <dd>Determines the ordering in which the overlays are
+          applied. Those with high priority are applied after those
+          with lower priority, and able to override the opaqueness of
+          the ones that come before. Defaults to 0.</dd>
+        </dl>
+      </dd>
+
       <dt id="removeOverlay"><code><strong>cm.removeOverlay</strong>(mode: string|object)</code></dt>
       <dd>Pass this the exact value passed for the <code>mode</code>
       parameter to <a href="#addOverlay"><code>addOverlay</code></a>,

lib/codemirror.js

@@ -4952,7 +4952,10 @@
     addOverlay: methodOp(function(spec, options) {
       var mode = spec.token ? spec : CodeMirror.getMode(this.options, spec);
       if (mode.startState) throw new Error("Overlays may not be stateful.");
-      this.state.overlays.push({mode: mode, modeSpec: spec, opaque: options && options.opaque});
+      insertSorted(this.state.overlays,
+                   {mode: mode, modeSpec: spec, opaque: options && options.opaque,
+                    priority: (options && options.priority) || 0},
+                   function(overlay) { return overlay.priority })
       this.state.modeGen++;
       regChange(this);
     }),
@@ -8370,6 +8373,13 @@
     return out;
   }
 
+  function insertSorted(array, value, score) {
+    console.log(array)
+    var pos = 0, priority = score(value)
+    while (pos < array.length && score(array[pos]) <= priority) pos++
+    array.splice(pos, 0, value)
+  }
+
   function nothing() {}
 
   function createObj(base, props) {