Specify random number generator

The code relies on `Math.random()`, which is fine, but this does not support seeded random numbers, for example [seedrandom](https://www.npmjs.com/package/seedrandom).

Proposed change:

```js
const perlin = require('perlin-noise');
const seedrandom = require('seedrandom');

const rng = seedrandom('hello.');

perlin.setRNG(rng);
```

A function with signature:

```ts
type RNG = (): number;

function setRNG(rng: RNG): void;
```

The default value would simply be `Math.random`.