Add option to fail license checks for UNKNOWN licenses

[joschi]

Sometimes it's desired to fail builds if the license of one of the dependencies is unknown.

This is not the same as "Pessimistic mode" which will fail builds if a dependency has multiple licenses and one of those is not on the whitelist.

[joschi]

I failed to realize that the licenseCheckBreakByUnknown option for the VersionEye Maven plugin can control this behavior.

[Robert Reiz]

Yes :-)