Eugene Yokota cd9f0d2711 make sbt server extensible ...

Fixes #3890

Here's an example:

```scala
    Global / serverHandlers += ServerHandler({ callback =>
      import callback._
      import sjsonnew.BasicJsonProtocol._
      import sbt.internal.protocol.JsonRpcRequestMessage
      ServerIntent(
        {
          case r: JsonRpcRequestMessage if r.method == "lunar/helo" =>
            jsonRpcNotify("lunar/oleh", "")
            ()
        },
        PartialFunction.empty
      )
```

2018-03-13 23:02:45 +09:00

..

src/main

make sbt server extensible

2018-03-13 23:02:45 +09:00