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 
@mikesherov While I like the idea of having other reporters pick up the maxError error message, the output doesn't look sensible: 

![image](https://cloud.githubusercontent.com/assets/865203/4536923/90c8df4a-4dd0-11e4-9e3e-702656b8c6cf.png)

I'd have to modify `explainError` to avoid printing anything but the maxError message. Plus, the count of errors would be wrong.

I still believe that the message should be either inline or after the `2 code style errors found.` message. The non-invasive alternative is to set a property on the checker once maxErrors has been hit and then console.log an additional message within `cli.js` after the errors are sent to the reporter.