Restore -XX:MaxPermSize.

Looks like I'll live with the warning on java8, because
sbt is unusable without this option on java7.
Unreleased version 0.13.5-RC1.
Capitalize on unknown sbt feature.

Nobody ever told me you can pass --error, --warn etc to sbt.
That's a big improvement because you can actually suppress all
the noise on startup.

Reworked the command line options a bit to reflect this.