feat: support custom contexts, WriteJSON, and Pipeline (#9)

* feat: support custom contexts, WriteJSON, and Pipeline

* test: improve unit test coverage

feat: log msg with no args without format (#8)

* Take msg as format string only if args are provided

* style: minor improvements

* test: improve unit tests coverage

* fix: unit test failure

feat: set custom writer in logger (#7)

* set and get output helper methods

* better comments

* corrected get

* dumb fix

* some tests

* @jlorgal suggestions

* updating comments...

feat: log timestamp in RFC3339 with milliseconds (#6)

feat: variadic accepted mw (#5)

fix: minor style improvements