Load nbextensions from paths in frontend config

So that instead of manually editing custom.js, extensions can be specified in declarative config:

```
~/.ipython/profile_default/nbconfig$ cat notebook.json 
{"load_extensions": {"cite2c/main": true}}
```

To come (probably in a separate PR): APIs to manipulate these config files from Python, so code installing an extension can easily enable it as well.

To come somewhat later: UI for listing and enabling/disabling extensions.