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
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- [x] Delete power-assert-formatter related files
- [x] Change empower function signiture
- [x] Fake formatter for unit testing
- [x] Create power-assert-formatter module
- [x] Recover test coverage
- [x] Make formatter a function, not object
- [x] Do not provide `bind` for old browsers
- [x] Refactoring