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 
I don't like this, but it should adhere to your suggestions. It implies that the maxErrors is configurable on a per-instance basis – which it is not. In any case, it will hopefully suffice.