Add ModelEngine animation option

[Rubenicos]

Added modelengine-animation option specified on wiki

This pull request requires Auxilor/ModelEngineBridge#1 to be merged

Information

For those people that probably experiment bugs using idle, walk or some other animation supported by default on ModelEngine, DO NOT use those animation names for pets. Pets are made with armor stands, and for some reason ModelEngine doesn't play default state animations on armor stands.

Also, the used animation should have a loop.