[WIP] Extract formatter

Trying to extract power-assert-formatter out.

empower module does not provide default formatter from now on.
power-assert module will compose assert object/function and formatter using `empower` function.

TODO
----------
- [x] Delete power-assert-formatter related files
- [x] Change empower function signiture
- [x] Fake formatter for unit testing
- [ ] Create power-assert-formatter module
- [ ] Refactoring