Add 'handlers' option to configure scrolling methods

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

README.md

@@ -283,61 +283,58 @@ imgLoader.perfectScrollbar();
 
 perfect-scrollbar supports optional parameters.
 
+### handlers
+It is a list of handlers to use to scroll the element.  
+**Default**: `['click-rail', 'drag-scrollbar', 'keyboard', 'wheel', 'touch']`  
+**Disabled by default**: `'selection'`
+
 ### wheelSpeed
 The scroll speed applied to mousewheel event.  
-**Default: 1**
+**Default**: `1`
 
 ### wheelPropagation
 If this option is true, when the scroll reaches the end of the side, mousewheel event will be propagated to parent element.  
-**Default: false**
+**Default**: `false`
 
 ### swipePropagation
 If this option is true, when the scroll reaches the end of the side, touch scrolling will be propagated to parent element.  
-**Default: true**
+**Default**: `true`
 
 ### minScrollbarLength
 When set to an integer value, the thumb part of the scrollbar will not shrink below that number of pixels.  
-**Default: null**
+**Default**: `null`
 
 ### maxScrollbarLength
 When set to an integer value, the thumb part of the scrollbar will not expand over that number of pixels.  
-**Default: null**
+**Default**: `null`
 
 ### useBothWheelAxes
 When set to true, and only one (vertical or horizontal) scrollbar is visible then both vertical and horizontal scrolling will affect the scrollbar.  
-**Default: false**
-
-### useKeyboard
-When set to true, the scroll works with arrow keys on the keyboard. The element is scrolled only when the mouse cursor hovers the element.  
-**Default: true**
+**Default**: `false`
 
 ### suppressScrollX
 When set to true, the scroll bar in X axis will not be available, regardless of the content width.  
-**Default: false**
+**Default**: `false`
 
 ### suppressScrollY
 When set to true, the scroll bar in Y axis will not be available, regardless of the content height.  
-**Default: false**
+**Default**: `false`
 
 ### scrollXMarginOffset
 The number of pixels the content width can surpass the container width without enabling the X axis scroll bar. Allows some "wiggle room" or "offset break", so that X axis scroll bar is not enabled just because of a few pixels.  
-**Default: 0**
+**Default**: `0`
 
 ### scrollYMarginOffset
 The number of pixels the content height can surpass the container height without enabling the Y axis scroll bar. Allows some "wiggle room" or "offset break", so that Y axis scroll bar is not enabled just because of a few pixels.  
-**Default: 0**
+**Default**: `0`
 
 ### stopPropagationOnClick
 When set to false, when clicking on a rail, the click event will be allowed to propagate.  
-**Default: true**
-
-### useSelectionScroll
-When set to true, you can scroll the container by selecting text and move the cursor.  
-**Default: false**
+**Default**: `true`
 
 ### theme
 A string. It's a class name added to the container element. The class name is prepended with `ps-theme-`. So default theme class name is `ps-theme-default`. In order to create custom themes with scss use `ps-container($theme)` mixin, where `$theme` is a scss map.  
-**Default: 'default'**
+**Default**: `'default'`
 
 **Example 1:**