Sbt plugin should export dependency versions as settings for use in examples and user code

[michaelpnash]

It is currently hard for users to track which versions of dependencies they are getting and which versions they should use when adding extra dependencies to their projects. This results in code like this in our own example projects:

libraryDependencies ++= Seq(
        "com.lightbend.akka"     %% "akka-stream-alpakka-file"  % "1.1.2",
        "com.typesafe.akka"      %% "akka-http-spray-json"      % "10.1.10",
        "ch.qos.logback"         %  "logback-classic"           % "1.2.3",
        "com.typesafe.akka"      %% "akka-http-testkit"         % "10.1.10" % "test",

We should update the sbt plugin to export its dependency versions so our example projects (and end user code) looks like this:

libraryDependencies ++= Seq(
        "com.lightbend.akka"     %% "akka-stream-alpakka-file"  % "1.1.2",
        "com.typesafe.akka"      %% "akka-http-spray-json"      % Cloudflow.AkkaHttpVersion,
        "ch.qos.logback"         %  "logback-classic"           % "1.2.3",
        "com.typesafe.akka"      %% "akka-http-testkit"         % Cloudflow.AkkaHttpVersion % "test",