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 
rather than switching to a for loop with break, we could just switch to `Array.prototype.some`, and return `true` inside the function, when max is hit. You can assign the result to `maxErrorsHit`, and use that to append the "max errors hit message".