1.3.0: Support Context Receivers for advanced usage

Now supports things like this:
```
dispatcher {
  "literal_arg_1" {
    executesX {
      // ... do something
    }

    "typed_arg_1"(StringArgumentType.greedyString()) {
      executesX {
        val theString = StringArgumentType.getString(it, "typed_arg_1")
        println(theString)
      }
    }
  }
}
}
```