Read dynamic compiler trait options from scalacOptions.

Now there isn't any reference to Scala.js in particular
anymore. Client build.sbt files can specify the dynamic
compiler trait they want to use and its jar.

This also has another advantage: since these things are
part of the compiler's command line, a different cached
compiler will be created for a different dynamic compiler
trait (or for none at all).

Typical usage from client build.sbt:

scalacOptions ++= Seq(
    "-Dsbt.compile.interface.dynamiccompilertrait=scala.tools.nsc.scalajs.JSGlobal",
    "-Dsbt.compile.interface.dynamiccompilerjar=/home/doeraene/.ivy2/local/ch.epfl.lamp/scala.js-compiler_2.10/0.1-SNAPSHOT/jars/scala.js-compiler_2.10.jar"
)

Open question: use -toolcp instead of
-D...dynamiccompilerjar?