Added threat indicator functionality to nameplates.

As requested in https://github.com/vikinghug/VikingNameplates/issues/11

I've added threat functionality to the nameplaces, currently this can be enabled by opening up the options and enabling it in the individual settings.

Preview
http://i.imgur.com/D0kNtsX.jpg

The threat indicator design isn't final i'll leave this up to design team.

Testing would be great :)