Adding some multiplicative functions

I am considering adding some basic prime-factorization-related functions to 'Primes.jl', in particular some [multiplicative functions]([https://en.wikipedia.org/wiki/Multiplicative_function](https://en.wikipedia.org/wiki/Multiplicative_function)) (in the sense of number theory, i.e, such that f(a*b) = f(a)f(b) if a and b are relative prime).

Primes.jl already has *totient* (but it does not appear to be documented; I might fix this). I would add at least the Möbius function, the divisor function, and the [Liouville function](https://en.wikipedia.org/wiki/Liouville_function). Some of these functions I have used to help solve Project Euler problems. In Python, such functions are provided by [SmyPy ](https://docs.sympy.org/latest/modules/ntheory.html).

Similar to the totient function I would add two versions each, to have efficiency in case the
factorization is already known, e.g.:
~~~Julia
moebiusmu(f::Factorization{T}) where T <: Integer
moebiusmu(n::Integer)
~~~

Also, I would add `divisors(n::Integer)` and `divisors(f::Factorization{T})` which return an array (or an iterator) giving all the positive divisors of n.

What do you think about this?