Add npc.is_engaged

A quick tag to determine whether an NPC is considered to be 'engaged'
(It appears somebody just assumed this existed
when really it didn't 0.o)
Plus, more meta-interlinking! Hooray!
Merge pull request #666 from mcmonkey4eva/master

Pull request for the dark lord of the underworld
Add <&hrt> for heart symbol

Because ♥ BlackCoyote
Merge pull request #668 from Morphan1/master

♥'s for all
Merge pull request #667 from Jeebiss/master

added EntityPowered mechanism
Merge branch 'master' of https://github.com/aufdemrand/Denizen