[dub] specify dependency configuration

[cal via Digitalmars-d-learn]

Given myapp and a dependency, specified by dub.json's:

myapp: dub.json
{
   ...
   "dependencies": {
     "dependency_a": ">=0.6.0"		
   }
   ...
}

dependency_a: dub.json
{
   ...
   "configurations": [
   {
     "name": "config_a",
     "targetType": "library",
     ...
   },
   {
     "name": "config_b",
     "targetType": "executable",
     ...
   },
   ...
}

How do I specify (in myapp: dub.json) that I want the "config_a" 
configuration of dependency_a?