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 
@markelog @mdevils @mikesherov Opted for a closed-over counter variable. Since we don't really enforce a default value for maxError anymore, I don't need to dangle a bunch of things on the constructor.