Feature/slot scheme with condition

[hyzhak]

#72

we could have an array of slot schemes now. And with conditions. 1st scheme without conditions or with conditions which match expressions with slots values will be chosen.

We use mathjs.org for comparisons here

[{
  // will be used only when slots.category is equal to "plates"
  conditions: [
    'category == "plates"'
  ],

  name: 'plates',

  slots: [
    'plate',
    'category',
  ],

  prompts: [{
    requirements: [
      'plate'
    ],

    prompts: [
      'Which plate?',
    ],
  }]
}, {
  name: 'albums',

  slots: [
    'album',
    'category',
  ],

  prompts: [{
    requirements: [
      'album'
    ],

    prompts: [
      'Which album?',
    ],
  }],
}];