Allow substitutions in keys

[danielyli]

Lightbend’s Cinnamon configuration for Akka HTTP uses user-configurable key names that need to match values in other parts of the configuration. It would be very helpful to support the following syntax:

app {
  http {
    host = "localhost"
    port = 8000
  }
}

cinnamon.akka.http.servers {
  ${app.http.host}.${app.http.port} {
    // etc...
  }
}

[havocp]

Somewhat related are #122 and #134 ... I felt like there might be an issue for this already but maybe i was thinking of those two slightly different ideas.

This is maybe trickier to specify than it might seem at first, in particular the interaction with merging (represented in code by the withFallback operation) is not immediately clear to me. Once specified it could also be tricky to implement - there's more to the substitution algorithm than one might expect. See https://github.com/lightbend/config/blob/master/HOCON.md#substitutions for the prose version or the code for the code version...

Just speculation though; it's possible that the existing rules for unresolved values could simply work for this case also (an object is unresolved if it contains anything unresolved, etc), if so maybe it doesn't add much more difficulty to do this. Needs exploration.