Add custom events for repositioning, shown, hidden

If you run some script (eg. locally or via an autosize plugin) which automatically resizes a textarea to fit its contents, then the maxlength indicator will be positioned incorrectly. This pull request binds a handler to a custom `maxlength.reposition` event which will reposition the indicator.

It also adds a couple of events which are triggered after showing/hiding the indicator which allow you to bind handlers outside the plugin.