Enable the continuations plugin.

With the minimal amount of contortions.

We bypass the `autoCompilerPlugins` feature of SBT in favour
of directly building up `scalacOptions` based on the resolved
classpath, which contains the version of the continutions plugin
specified in scala-dist's POM.

I've introduced a key for the scala-dist version, rather
than using `version` directly. This was mostly so I could
test this locally, and probably isn't the right way. But
the README needs to be fleshed out to guide me down the
right path.