Add custom layer option to URL hash, fixes #1024.

Layer format is custom:http://url.com/{z}/{x}/{y}.jpg

js/id/id.js

@@ -133,7 +133,14 @@ window.iD = function () {
     };
 
     var q = iD.util.stringQs(location.hash.substring(1)), detected = false;
-    if (q.layer) {
+    if (q.layer && q.layer.indexOf('custom:') === 0) {
+        context.layers()[0]
+           .source(iD.BackgroundSource.template({
+                template: q.layer.replace(/^custom:/, ''),
+                name: 'Custom'
+            }));
+        detected = true;
+    } else if (q.layer) {
         context.layers()[0]
            .source(_.find(backgroundSources, function(l) {
                if (l.data.sourcetag === q.layer) {

js/id/renderer/background.js

@@ -163,6 +163,9 @@ iD.Background = function(backgroundType) {
 
     function setHash(source) {
         var tag = source.data && source.data.sourcetag;
+        if (!tag && source.data.name === 'Custom') {
+            tag = 'custom:' + source.data.template;
+        }
         var q = iD.util.stringQs(location.hash.substring(1));
         if (tag) {
             q[backgroundType] = tag;