Warn about .py/.json collisions in config...
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
