Asynchronous PRNG (and .gen() method)

I want to use an async function to generate cryptographically strong random numbers for Randexp. But seems like currently `randexp.randInt` can only return generated value, not a `Promise` of the value. It would be good to see that implemented so that I can generate cryptographically strong passwords and tokens according to regular expressions. Although making that possible probably also means making `.gen()` function async too...