feat(message): implement the new @formatMessage method

See AtomLinter/linter-jshint#27
Specify command as array
Update README.md
Prepare 0.0.18 release
Merge branch 'stephanebachelier-format-message'
Infer type from match.{error,warning}

Test Plan:

  * Ran lint, saw it display type
  * Wrecked the regex and then saw lint report an error