CLI: Support a maxErrors option to limit the number of reported errors

- [x] Add info to readme
- [x] Test config version of maxErrors
- [x] Test default value (50) in lib/errors

Fixes #238 
Yup, setting the property on the checker works well:

![image](https://cloud.githubusercontent.com/assets/865203/4536986/dd1bc082-4dd1-11e4-8628-715b9718489d.png)