Simplify specifying custom directives from sbt

[jrudolph]

We need a custom directive for akka-http for a customization of @snip to include scala signatures from files. This is possible to do by adapting the paradoxProcessor key like this:

  val paradoxWithSignatureDirective = Seq(
    paradoxProcessor in Compile ~= { _ =>
      // FIXME: this is a HACK so far that copies stuff over from paradox
      // it would be better if the plugin has a way of specifying extra directives through normal sbt mechanisms
      new ParadoxProcessor(writer =
        new Writer(serializerPlugins = context =>
          Seq(
          new ActiveLinkSerializer,
          new AnchorLinkSerializer,
          new DirectiveSerializer(Writer.defaultDirectives(context) :+
            new SignatureDirective(context.location.tree.label) // <-- only line that is no boilerplate
      ))))
    }
  )

It would be nice if custom directives could be specified by just adding them to key.