sbtconfig takes external SBT_OPTS

```
% bash -x ./sbt compile
...
++ PERM_OPT=-XX:MaxMetaspaceSize=386M
++ export 'SBT_OPTS=-XX:+CMSClassUnloadingEnabled -XX:MaxMetaspaceSize=386M '
++ SBT_OPTS='-XX:+CMSClassUnloadingEnabled -XX:MaxMetaspaceSize=386M '
+ exec java -Xmx1024M -XX:+CMSClassUnloadingEnabled -XX:MaxMetaspaceSize=386M -jar /skinny-app/bin/sbt-launch.jar compile


% export SBT_OPTS=-Dhoge=foo
% bash -x ./sbt compile
...
++ PERM_OPT=-XX:MaxMetaspaceSize=386M
++ export 'SBT_OPTS=-XX:+CMSClassUnloadingEnabled -XX:MaxMetaspaceSize=386M -Dhoge=foo'
++ SBT_OPTS='-XX:+CMSClassUnloadingEnabled -XX:MaxMetaspaceSize=386M -Dhoge=foo'
+ exec java -Xmx1024M -XX:+CMSClassUnloadingEnabled -XX:MaxMetaspaceSize=386M -Dhoge=foo -jar /skinny-app/bin/sbt-launch.jar compile
```