Make dependency on activity indicator plugin optional

I've run into big issues with test automation after adding chatjs to my project. The reason is the dependency on the activity indicator plugin. It turns out it's not supported by HtmlUnit (see the issue I opened at https://sourceforge.net/p/htmlunit/bugs/1538/).

The simplest way to solve it for me was to disable the call to activity plugin and style the loading icon myself. I think it makes sense to allow users to use their own loading icon that can be made consistent with loading icons used in other places in the application. 

I modified the plugin code and added new parameter "useActivityIndicatorPlugin". By default it behaves like today but you can set this option to false and get rid of activity indicator plugin and style the loading box yourself in CSS. 

Could you merge this change?