Dev flags: merge maps with same name defined in different config files instead of overwriting

This will be used shortly for advanced configuring of Bower.

E.g.
app.json:
  "some-map": {
    "a": "aa"
  }

.spark.json:
  "some-map": {
    "b": "bb"
  }

will result in:

  "some-map": {
    "a": "aa",
    "b": "bb"
  }
Merge pull request #2233 from ussuri/improve-dev-flag-parsing

Dev flags: merge maps with same name defined in different config files instead of overwriting