Adding option to disable "PASS" messages for lint

When one has hundreds of small JavaScript files that get linted, seeing
pages and pages of success messages starts to hide the errors.
Eliminating output when there is no problem makes the lint violations
stand out more prominently.

This option is disabled by default to preserve the original behavior.
Merge pull request #5 from fidian/errors-only

Adding option to disable "PASS" messages for lint