Add bang support to :delgroup to delete all groups.

common/locale/en-US/repeat.xml

@@ -189,10 +189,13 @@
 </item>
 
 <item>
+    <tags>:delgr :delgroup</tags>
     <spec>:delgr<oa>oup</oa> <a>group</a></spec>
+    <spec>:delgr<oa>oup</oa>!</spec>
     <description>
         <p>
-            Delete the specified <a>group</a>.
+            Delete the specified <a>group</a>. With <oa>!</oa> delete all
+            user groups.
         </p>
     </description>
 </item>

common/modules/contexts.jsm

@@ -570,12 +570,21 @@ var Contexts = Module("contexts", {
         commands.add(["delg[roup]"],
             "Delete a group",
             function (args) {
-                util.assert(contexts.getGroup(args[0]), _("group.noSuch", args[0]));
-                contexts.removeGroup(args[0]);
+                util.assert(args.bang ^ !!args[0], _("error.argumentOrBang"));
+
+                if (args.bang)
+                    contexts.groupList = contexts.groupList.filter(function (g) g.builtin);
+                else {
+                    util.assert(contexts.getGroup(args[0]), _("group.noSuch", args[0]));
+                    contexts.removeGroup(args[0]);
+                }
             },
             {
-                argCount: "1",
+                argCount: "?",
+                bang: true,
                 completer: function (context, args) {
+                    if (args.bang)
+                        return;
                     modules.completion.group(context);
                     context.filters.push(function ({ item }) !item.builtin);
                 }