Added support for custom events via shortcode filters.

I've developed a small patch (against current trunk version) that allows you to use shortcodes (both in articles/pages and text widgets) to create custom Google Analytics events for links (both internal and external).

Using the shortcodes you can specify any of the Google Analytics events' details: category, action, label, value and noninteractio)n with the following syntax:

    [wga_event category="My Category" action="My Action"] an html link [/wga_event]

If you don't specify the category and action values, default values are used (as Google Analytics consider them mandatory).

Signed-off-by: Iñaki Arenaza <iarenaza@mondragon.edu>