Do not fail if no source was found

[filiphr]

This is linked with #57.

I would like to have an option to emit a warning or ignore if a source could not be found. Currently I have to exclude all the files that I don't want in the report in the Jacoco plugin.

The use case is to not include generated files in the coverage report. Jacoco per default uses all the class files in the output directory to create the report, and the coveralls-maven-plugin uses the sources.

If you are interested in something like this I can have a look and try to add the functionality.

[ebcodes]

+1. Would love this capability.