[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?