Add Generex#random(RandomSource random) so random source can be provided on a call-by-call basis

[vlsi]

Currently it is hard to integrate Generex and https://github.com/pholser/junit-quickcheck.

Here's "example" implementation: https://github.com/pholser/junit-quickcheck/blob/61e751ceb55cb767d7ef150959df0244543fcd64/examples/src/test/java/com/pholser/junit/quickcheck/examples/text/Structured.java#L51

There are two problems there:

1. Generex(pattern) initialization is performed on each generation. In other words, Generex parses the pattern and builds regex automata for each generate call which is waste.

2. Cloning Random is kind of hard. In practice, Generex#prepareRandom does not need the full power of Random, so it might make sense to add a thin interface RandomSource with methods like nextInt(int) and nextFloat().

Then end-users could instantiate Generex just once, and pick random values as needed (providing it with RandomSource instance).