_logIf => _log
find all `util.inspect` and if it's an error, pass to `_logIf` directly
leading tabs to spaces
accept third param as error, not connection (replace `': ' + err`)
eslint:
  should warn on `x === null`
move leftPad, DOES_NOT_REQUIRE_AUTH, REQUIRES_CONFIGURED_DATA

add `conn` listeners in FtpConnection constructor

re-implement pr/53 (Enhance MDTM feature) and add/fix tests
  https://github.com/sstur/nodeftpd/pull/53

pasv refactor:
  make `pasv` (tcp server) a class with method that will stop listening (close) and destroy any existing connections.
  add test for: https://github.com/sstur/nodeftpd/pull/34
  abstract out the port-finding logic in `_setupNewPASV`