feat(onelog): add With method to add fields to the logger

fix(tests): fill in tests and remove obsolete nil check from methods

fix(onelog): add tests and fix bugs

fix(adapter/slog): fix badkey bug

docs(examples): add _examples folder

docs(onelog): update usage example

feat(onelog): add TimeDiff option

feat(adapter): add nop adapter

feat(onelog): init project