Adding the sbt-extras shell that takes care of the SBT environment.

Provide a shell that takes care of bootstrapping the SBT environment
and, as a side effect, enhance it. Quoted from the
[sbt-extars project](https://github.com/paulp/sbt-extras):

    An alternative script for running sbt. It works with sbt 0.13.0 projects
    and (in principle) all earlier versions. If you're in an sbt project
    directory, the system will figure out the required versions of sbt and
    scala, downloading them if necessary.