Double factorial can be extended to negative ints

[ron-wolf]

The following error may be averted, or such functionality made possible with an additional method:

Combinatorics.jl/src/factorials.jl

Lines 47 to 54 in c2114a7

	function doublefactorial(n::Integer)
	if n < 0
	throw(DomainError(n, "n must be nonnegative"))
	end
	z = Ref{BigInt}(0)
	ccall((:__gmpz_2fac_ui, :libgmp), Cvoid, (Ref{BigInt}, UInt), z, UInt(n))
	return z[]
	end

An example of how ‼︎ can be extended to the signed integers is found on Wikipedia at Ellipse § Circumference:

where $n‼︎$ is the double factorial (extended to negative odd integers 
by the recurrence relation $(2n-1)‼︎ = (2n+1)‼︎ / (2n+1)$, for $n ≤ 0$).

Would this be a useful addition to the library? Alternatively, if there are multiple distinct analytical continuations of ‼︎ for the negative integers, could I have a lead for how to implement it in Julia? Would this issue be a better fit for the SpecialFunctions library?