Allow client to control mapping between urls and files

For example:

    class WorkbenchRouting extends com.lihaoyi.workbench.Routing {
      def route(app: Directives)
               (implicit
                settings: RoutingSettings,
                resolver: ContentTypeResolver,
                refFactory: ActorRefFactory,
                log: LoggingContext): Route = {
        app.pathPrefix("css") {
          app.getFromDirectory(s"ui/target/web/sass/main/")
        }
      }
    }

    routing := new WorkbenchRouting,

src/main/scala/workbench/Plugin.scala

@@ -16,6 +16,7 @@ object Plugin extends sbt.Plugin {
   val updateBrowsers = taskKey[Unit]("Partially resets some of the stuff in the browser")
   val spliceBrowsers = taskKey[Unit]("Attempts to do a live update of the code running in the browser while maintaining state")
   val localUrl = settingKey[(String, Int)]("localUrl")
+  val routing = settingKey[Routing]("route")
   private[this] val server = settingKey[Server]("local websocket server")
 
 
@@ -95,7 +96,8 @@ object Plugin extends sbt.Plugin {
         }
       }
     },
-    server := new Server(localUrl.value._1, localUrl.value._2, bootSnippet.value),
+    routing := DefaultRouting,
+    server := new Server(localUrl.value._1, localUrl.value._2, bootSnippet.value, routing.value),
     (onUnload in Global) := { (onUnload in Global).value.compose{ state =>
       server.value.kill()
       state

src/main/scala/workbench/Routing.scala

@@ -0,0 +1,26 @@
+package com.lihaoyi.workbench
+
+import akka.actor.ActorRefFactory
+import spray.routing._
+import spray.routing.directives.ContentTypeResolver
+import spray.util.LoggingContext
+
+trait Routing {
+  def route(app: Directives)
+           (implicit
+            settings: RoutingSettings,
+            resolver: ContentTypeResolver,
+            refFactory: ActorRefFactory,
+            log: LoggingContext): Route
+}
+
+object DefaultRouting extends Routing {
+  def route(app: Directives)
+           (implicit
+            settings: RoutingSettings,
+            resolver: ContentTypeResolver,
+            refFactory: ActorRefFactory,
+            log: LoggingContext): Route = {
+    app.getFromDirectory(".")
+  }
+}

src/main/scala/workbench/Server.scala

@@ -27,7 +27,7 @@ import scala.tools.nsc.typechecker.Analyzer
 import org.scalajs.core.tools.classpath.{CompleteClasspath, PartialClasspath}
 import scala.tools.nsc.util.{JavaClassPath, DirectoryClassPath}
 
-class Server(url: String, port: Int, bootSnippet: String) extends SimpleRoutingApp{
+class Server(url: String, port: Int, bootSnippet: String, routing: Routing) extends SimpleRoutingApp{
   implicit val system = ActorSystem(
     "Workbench-System",
     config = ConfigFactory.load(ActorSystem.getClass.getClassLoader),
@@ -111,7 +111,7 @@ class Server(url: String, port: Int, bootSnippet: String) extends SimpleRoutingA
           """
         }
       } ~
-      getFromDirectory(".")
+      routing.route(this)
 
     } ~
     post {