Weighted distribution

[guyb7]

Add an option for choosing a custom distribution.

Specify a weight percentage for each variant:

app.use(split({
    experiments: {
      'price': {options: [
        {name: '100', weight: 0.15},
        {name: '250', weight: 0.8},
        {name: '400', weight: 0.05}
      ]
    }
}}));