Allow the 'next' property of a keymap to be a function

It'll be called with the current instance as argument, and should
*always* return a string that refers to a keymap.

Experimental feature, not documented yet.

lib/codemirror.js

@@ -444,8 +444,11 @@ var CodeMirror = (function() {
     }
     function handleKeyBinding(e) {
       var name = keyNames[e.keyCode], next = keyMap[options.keyMap].auto, bound, dropShift;
+      function handleNext() {
+        return next.call ? next.call(null, instance) : next;
+      }
       if (name == null || e.altGraphKey) {
-        if (next) options.keyMap = next;
+        if (next) options.keyMap = handleNext();
         return null;
       }
       if (e.altKey) name = "Alt-" + name;
@@ -460,7 +463,7 @@ var CodeMirror = (function() {
         if (commands.propertyIsEnumerable(bound)) bound = commands[bound];
         else bound = null;
       }
-      if (next && (bound || !isModifierKey(e))) options.keyMap = next;
+      if (next && (bound || !isModifierKey(e))) options.keyMap = handleNext();
       if (!bound) return false;
       if (dropShift) {
         var prevShift = shiftSelecting;