#137 Adding external message listener

[JasperTack]

By adding this listener for external messages, we allow for other extensions or web pages themselves to send messages to the SabConnect++ extension.

This is for example useful when sabnzbd links are injected dynamically by another extension. Since the links on a webpage are only analysed once right after the page is loaded, any content that has been inject dynamically at a later stage won't be analysed and won't be sent to the SabConnect++ extension. Therefore, it would be useful if the other extension is able to send the 'addToSabnzbd' message manually to the SabConnect++ extension.

[vanilla-thunder]

otherwise you could add event listenes like this:
$("body").on("click",$element, function() { add_link_to_sabnzbd(); });
instead of placing event listenes on buttons / links itself