Added support for setting random number generator, to make Tracery deterministic

[jhorneman]

As briefly discussed on Twitter.

I've put Math.random in a variable, made Tracery use that instead of direct calls to Math.random, then added a function setRng to set that variable with a different function.

I've used this in the tests to be sure it worked, but you might not want that. Up to you: it's a tricky question.

I've documented the function in the readme file.