Allow custom project-specific settings (such as formatter.rewriteArrowSymbols)

[raboof]

I'd like to add some settings to .settings/org.scala-ide.sdt.core.prefs.

A glance at /src/main/scala/com/typesafe/sbteclipse/core/Eclipse.scala suggests that this is only possible via the sbt.Keys.scalacOptions (and even then only for those options supported by fromScalacToSDT in /src/main/scala/com/typesafe/sbteclipse/core/EclipseSDTConfig.scala)

It would be nice to add custom keys such as formatter.rewriteArrowSymbols .