Add ability to specify MP type

[parnic]

This enables asking for MP(type="energy") while in Bear form as a Druid, for example, without having to add Energy-specific tags.

This should probably turn the Druid and Monk specific tags into aliases, but I don't want to change their behavior since they specifically return nil if you aren't the appropriate class.

[parnic]

IceHUD's use case is described here: https://github.com/parnic/ice-hud/commit/d775603ec0af481857001b92c5781f791a4aa020