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'd add an error here, so that all reporters get the error saying the max was hit. Then I'd add another conditional for `Errors._errorCount > Errors.maxErrors` to `return false`, so that the "max error" error only gets added once ever.