sbt new should launch sbt after creating project

Originally reported as https://github.com/sbt/sbt/issues/2840

## steps
```
$ sbt new $someTemplate
```

## problem
Once this is done, need to cd into the new directory (first need to figure out what it's called) and run sbt again.


## expectation
I end up in an sbt shell immediately after `sbt new` completes.
