Add --relative_files parameter to xml report

[Ar-b-ra]

Slipcover generates Cobertura XML output file. It's very convenient, so i can use in Cobertura plugin in Jenkis.

But, i can't use it to show coveraged lines in report because of relative paths in report.

I solve it like this:

sh 'sed -i "s|src.||g" ./coverage.xml'
sh 'sed -i "s|<source>/app</source>|<source>src</source>|g" ./coverage.xml'

But, it could be better to have an option to write absolute path in report.
Coverage uses relative_files parameter.

Could you add it to project?