[sbt] Allow the REPL to be run from the SBT build

  - Tell SBT to that we're forking an interactive process
  - Automatically add `-usejavacp` so the REPL adds the classes
    from the system classloader to the compilers classpath.

JLine seems to be working from within this setup.

```
% sbt
Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=384m; support was removed in 8.0
[info] Loading global plugins from /Users/jason/.sbt/0.13/plugins
[info] Loading project definition from /Users/jason/code/scala2/project
[info] *** Welcome to the sbt build definition for Scala! ***
[info] This build definition has an EXPERIMENTAL status. If you are not
[info] interested in testing or working on the build itself, please use
[info] the Ant build definition for now. Check README.md for more information.
> repl/run
[info] Running scala.tools.nsc.MainGenericRunner -usejavacp
Welcome to Scala version 2.11.6-SNAPSHOT-20150528-131650-70f0b1ded8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_25).
Type in expressions to have them evaluated.
Type :help for more information.

scala> 1 + 1
res0: Int = 2

(reverse-i-search)`1': 1 + 1
```

Author: retronym

build.sbt

@@ -192,7 +192,12 @@ lazy val interactive = configureAsSubproject(project)
   .dependsOn(compiler)
 
 lazy val repl = configureAsSubproject(project)
-  .settings(libraryDependencies += jlineDep)
+  .settings(
+    libraryDependencies += jlineDep,
+    connectInput in run := true,
+    outputStrategy in run := Some(StdoutOutput),
+    run <<= (run in Compile).partialInput(" -usejavacp") // Automatically add this so that `repl/run` works without additional arguments.
+  )
   .settings(disableDocsAndPublishingTasks: _*)
   .dependsOn(compiler)