Passing AbstractRNG to rand

[theogf]

Right now it's not possible to pass a random number generator to rand(::Type{Colorant}).
Would you be open to a PR fixing this reproducibility issue by adding rand(::AbstractRNG, ::Type{Colorant}) and co?

[theogf]

@johnnychen94 I think this is already fixed in the dev version of 0.12, would it be possible to get a release?

[johnnychen94]

v0.12 seems not finished according to @kimikage's plan, it would be great to have some new love to finish it

I get too busy these days to contribute; really sorry about that.

[kimikage]

It could be backported to v0.11.
cf. #299