Only fail build on errors and allow lower severities to pass

[campbes]

Add option so that complicated functions that have a lower severity than error will not fail the build.

I would have usually used a "breakOnlyOnError" type option instead of "allowWarnings", but both "breakError" and "errorsOnly" are already in use. Redefining those would cause backwards-compat issues, hence the slightly oddly named allowWarnings. Defaults to false to maintain existing behaviour.