Custom mainclass

The PR allows for a custom main class as an arg to the bash script. The primary motivation for this is adding the ability to run something like the following to get a repl for an app:

```
./target/universal/stage/bin/my-app -main scala.tools.nsc.MainGenericRunner -usejavacp
```
The above requires adding the following to your `build.sbt` of course:

```
libraryDependencies <+= scalaVersion("org.scala-lang" % "scala-compiler" % _ )
```

If this PR is acceptable I will take the time to add this to the bat-template as well.