Improve `harmonic`

[axkr]

Sorry I somehow missed #18.

Can the formula: HarmonicNumber(n) == EulerGamma + PolyGamma(0, 1 + n)

be used in harmonic for non-integer arguments?

https://github.com/paulmasson/math/blob/e8a5603a3157d0ff044b8126f3bca0d3c5bcad26/build/math.js#L4464

Also for 2 arguments: HarmonicNumber(n, r) == -HurwitzZeta(r, 1 + n) + Zeta(r)