Add isValue into mappers

[Siemienik]

Part of #21

TODO:

1. Create file src/mappers/isValue.ts with function matches to type (...acceptedValues:string[]) => ValueMapper<boolean>. Which check value to be included in accepted values. Returns false if no accepted values provided.
2. reexport it in src/mappers/index.ts
3. Prove that it works in unit test
4. Describe it in Readme.md

proposed test data provider:

const dataProvider = [
  // without accepted values
  { inValue: '', inAcceptedValue:[], expectedResult: false },
  { inValue: 'TRUE', inAcceptedValue:[], expectedResult: false },
  { inValue: 'X', inAcceptedValue:[], expectedResult: false },
  { inValue: '1', inAcceptedValue:[], expectedResult: false },
  { inValue: 'FALSE', inAcceptedValue:[], expectedResult: false },
  { inValue: 'Lorem', inAcceptedValue:[], expectedResult: false },

  // with one accepted value
  { inValue: '', inAcceptedValue:['X'], expectedResult: false },
  { inValue: 'TRUE', inAcceptedValue:['X'], expectedResult: false },
  { inValue: 'X', inAcceptedValue:['X'], expectedResult: true },
  { inValue: '1', inAcceptedValue:['X'], expectedResult: false },
  { inValue: 'FALSE', inAcceptedValue:['X'], expectedResult: false },
  { inValue: 'Lorem', inAcceptedValue:['X'], expectedResult: false },

  // with more than one accepted value
  { inValue: '', inAcceptedValue:['X', 'TRUE', '1'], expectedResult: false },
  { inValue: 'TRUE', inAcceptedValue:['X', 'TRUE', '1'], expectedResult: true },
  { inValue: 'X', inAcceptedValue:['X', 'TRUE', '1'], expectedResult: true },
  { inValue: '1', inAcceptedValue:['X', 'TRUE', '1'], expectedResult: true },
  { inValue: 'FALSE', inAcceptedValue:['X', 'TRUE', '1'], expectedResult: false },
  { inValue: 'Lorem', inAcceptedValue:['X', 'TRUE', '1'], expectedResult: false },

  // with different case
  { inValue: 'TRUE', inAcceptedValue:['true'], expectedResult: true },
  { inValue: 'X', inAcceptedValue:['x'], expectedResult: true },
];

For making a reservation please open draft PR with this issue linked