add support for system configuration of SBT installations

This pull request adds a system-wide configuration option to add parameters to the sbt call. For example, this can be used to change the `.sbt` directory for different installations of SBT.

Note that I have no idea how to write Jenkins plugins. I think I should have used `ToolProperty`, but I wasn't able to find an example of how to do it. If you know a better way of adding this functionality, please