Adding additional modules no longer possible

[johanandren]

          The fix makes things worse if you need to enable additional modules. In 0.4.6 I can add the `--add-modules` option to sbt itself (which needs them anyway for Zinc to work), but now that it runs forked, this no longer works and there doesn't seem to be any way to add the option to the forked process.

Originally posted by @szeiger in #190 (comment)