Start iD.operations.Merge (#435)

It's currently limited to merging (joining) exactly two lines.

Fixes #370.

js/id/operations/merge.js

@@ -0,0 +1,27 @@
+iD.operations.Merge = function(selection, context) {
+    var action = iD.actions.Join(selection[0], selection[1]);
+
+    var operation = function() {
+        context.perform(
+            action,
+            t('operations.merge.annotation', {n: selection.length}));
+    };
+
+    operation.available = function() {
+        return selection.length === 2 &&
+            _.all(selection, function (id) {
+                return context.geometry(id) === 'line';
+            });
+    };
+
+    operation.enabled = function() {
+        return action.enabled(context.graph());
+    };
+
+    operation.id = "merge";
+    operation.key = t('operations.merge.key');
+    operation.title = t('operations.merge.title');
+    operation.description = t('operations.merge.description');
+
+    return operation;
+};