Add 'handlers' option to configure scrolling methods

Please refer to README.md and examples/options-handlers.html for the
usage.

src/js/plugin/initialize.js

@@ -6,13 +6,15 @@ var cls = require('../lib/class')
   , updateGeometry = require('./update-geometry');
 
 // Handlers
-var clickRailHandler = require('./handler/click-rail')
-  , dragScrollbarHandler = require('./handler/drag-scrollbar')
-  , keyboardHandler = require('./handler/keyboard')
-  , mouseWheelHandler = require('./handler/mouse-wheel')
-  , nativeScrollHandler = require('./handler/native-scroll')
-  , selectionHandler = require('./handler/selection')
-  , touchHandler = require('./handler/touch');
+var handlers = {
+  'click-rail': require('./handler/click-rail'),
+  'drag-scrollbar': require('./handler/drag-scrollbar'),
+  'keyboard': require('./handler/keyboard'),
+  'wheel': require('./handler/mouse-wheel'),
+  'touch': require('./handler/touch'),
+  'selection': require('./handler/selection')
+};
+var nativeScrollHandler = require('./handler/native-scroll');
 
 module.exports = function (element, userSettings) {
   userSettings = typeof userSettings === 'object' ? userSettings : {};
@@ -25,21 +27,11 @@ module.exports = function (element, userSettings) {
   i.settings = h.extend(i.settings, userSettings);
   cls.add(element, 'ps-theme-' + i.settings.theme);
 
-  clickRailHandler(element);
-  dragScrollbarHandler(element);
-  mouseWheelHandler(element);
+  i.settings.handlers.forEach(function (handlerName) {
+    handlers[handlerName](element);
+  });
+
   nativeScrollHandler(element);
 
-  if (i.settings.useSelectionScroll) {
-    selectionHandler(element);
-  }
-
-  if (h.env.supportsTouch || h.env.supportsIePointer) {
-    touchHandler(element, h.env.supportsTouch, h.env.supportsIePointer);
-  }
-  if (i.settings.useKeyboard) {
-    keyboardHandler(element);
-  }
-
   updateGeometry(element);
 };