Add splitMapper into mappers

[Siemienik]

Part of #21

TODO:

1. Create file src/mappers/splitMapper.ts with function matches to type (v: string): string[]. Which for any input try to return only lower case string.
2. reexport it in src/mappers/index.ts
3. Prove that it works in unit test (create new one in tests/unit/mappers.test.ts)
4. Describe it in Readme.md

test data provider:

const lowerCaseDp = [
  { inValue: '', expectedResult: [] },
  { inValue: 'asd', expectedResult: ['asd'] },
  { inValue: 'asd,', expectedResult: ['asd',''] },
  { inValue: 'asd,asd2', expectedResult: ['asd','asd2'] },
  { inValue: 'true,1, expectedResult: ['true','1'] },
]

Further (not in scope of this issue)

• setting for separator
• setting for itemMapper