Please add (again) support for custom random sources

[jmwielandt]

Hi, basically this is a repost of #138. I can't access your functions defined at random_source.go and being able to pass my instance of math.Rand to the whole application (for having a log of seeds for future replication) is very important to me.

Thanks!

[bxcodec]

Hey @jmwielandt,
can you explain more what you need?

I thought the function on the random_source.go is a public function, and can be accessed from any project?