Request: get moves for piece

[DarthChucks]

I'd like to get a list of possible moves the piece on a particular space. Right now the "moves" function returns all possible moves for every piece of the color taking the turn. I can filter that list down to the piece I'm interested in, however it seems fairly inefficient to build the full list and then pare it back down.

User Story: As a player, I want to see all possible moves for the piece I've selected so that I can choose where to move the piece.

[jhlywa]

Added in 9d2489a. See https://github.com/jhlywa/chess.js#moves-options- for more info.