Allow placeholder in reportDir

[franzbecker]

Thanks for the great work!

I've got one issue with the configuration:

Currently I'm using this step to publish the scoverage results:

step([$class: 'ScoveragePublisher', reportDir: 'target/scala-2.12/scoverage-report', reportFile: 'scoverage.xml'])

What I really would want to configure is the following:

reportDir: 'target/scala-*/scoverage-report'

or

reportDir: 'target/**/scoverage-report' 

so that I don't need to update all my configurations when I upgrade the Scala version.
When I try this I get errors that the report cannot be found. Looks like this line right here:

scoverage-plugin/src/main/java/org/jenkinsci/plugins/scoverage/ScoveragePublisher.java

Line 64 in a963910

final FilePath scovPath = workspace.child(reportDir);

Or did I miss something?