FactorHistogram does not distinguish numbers that have no factorization

[gwhitney]

Currently, FactorHistogram treats 0, +- 1, and numbers without known factorizations all the same, as having 0 factors. I don't mind 0 and +-1 all being lumped together, but numbers of absolute value 2 or more, the factorizations of which are not known, should be treated differently. They certainly have some number of factors, we just don't know how many. So they should either be removed from the count altogether, or they should be in a separate "unknown" bar or be considered to have "-1" factors, or something along those lines.

[gwhitney]

This should be considered but optional for the alpha overhaul of FactorHistogram, so not setting a milestone here.