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 
Could you move that to it own module like `fileExtensions`, `excludeFiles` and `additionalRules`. 

Also you don't need to delete it, or add `_maxErrors` property see how we did modules above