Support extending dojorc configuration about cli

[JamesLMilner]

What level of config merging would we like to do here? I think top level property merging is an obvious requirement i.e.

// .dojorc
{
  "extends": "./base-dojorc",
  "build-app": {
    "legacy": false,
    "build-time-render": {
       // stuff
     }
  }
}

// .base-dojorc
{
  "pwa": {
    "manifest": {...},
  }
}

// Results in:

{
  "extends": "./base-dojorc",
  "build-app": {
    "legacy": false,
    "build-time-render": {
       // stuff
     }
  }
  "pwa": {
    "manifest": {...},
  }
}

However, if we have matching top level properties like so, how would we expect this to behave?

// .dojorc
{
 "extends": "./base-dojorc",
  "build-app": {
    "legacy": false,
    "build-time-render": {
       // child properties
     }
  }
}


// .base-dojorc
{
  "build-app": {
    "legacy": true,
    "build-time-render": {
        // child properties
     }
     "proxy": {
        // child properties
    }
}

// Results in?

Here the question is should it completely replace the whole build-app top level property, or should it try and merge it's child properties? If so, do we merge all the way down the tree or do we stop at the first level of children?

To me a sensible output for this situation would be :

{
 "extends": "./base-dojorc",
  "build-app": {
    "legacy": false,
    "build-time-render": {
       // .dojorc "build-time-render" properties
     }
    "proxy": {
        // .base-dojorc "proxy" properties
     }
  }
}