Add `--unix_mode` option for gjslint exec [#348]

Specify `--unix_mode` option, an error message will be output in UNIX
format. This format is including file-name every lines.

Previously, the error message did not include file-name. I think good
that specify this option.
Update changelog
[Fix #348] Merge branch 'gjlsint-unix_mode-option'

* gjlsint-unix_mode-option:
  Update changelog
  Add `--unix_mode` option for gjslint exec [#348]