Warn about .py/.json collisions in config

when two versions of the config file set the same value,
warn about clobbering, with a message, such as:

```
[TerminalIPythonApp] WARNING | Collisions detected in ipython_config.py and ipython_config.json config files. ipython_config.json has higher priority: {
  "Klass": {
    "trait": "'python' ignored, using 'json'"
  }
}
```


In anticipation of more machine-written .json config
Merge pull request #7082 from minrk/warn-collision

Warn about .py/.json collisions in config