Add pairwise ops.

Vector: pairwiseMultiply and pairwiseDivide. Matrix: pairwiseMultiply. In docs tagged as "Future".

Note that https://github.com/NaturalNode/node-sylvester has similar methods (albeit slightly slower implementations) with different names: elementMultiply, elementDivide. Wikipedia lists possible names as Hadamard product, Schur product, entrywise product (similar to pairwise) and googling seems more favorable for pairwise, but open to changing if we can sync code bases. (@chrisumbel)