Add dateMapper into mappers

[Siemienik]

Part of #21

TODO:

1. Create file src/mappers/dateMapper.ts with function matches to type ValueMapper<Date>. Which parse value string into Date.
2. reexport it in src/mappers/index.ts
3. Prove that it works in unit test (some present dates, some future, some past & some invalid)
4. Describe it in Readme.md

tips:

• ExcelJS used inside xlsx-import return Date strings created by new Date().toString()
  fi. Thu Oct 08 2020 02:00:00 GMT+0200 (Central European Summer Time)

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