Configure voila using traitlets configuration system

[maartenbreddels]

Needs rebasing after #83 is merged.
Will replace #80, by doing, e.g:

Example usage

~/.jupyter/voila.py

c.MappingKernelManager.cull_interval = 10

or, ${prefix}/etc/jupyter/voila.json

{
    "Voila": {
        "template": "gridstack"
    },
    "MappingKernelManager": {
        "cull_interval": 10
    }
}

Running voila with --Voila.log_level=DEBUG will show which paths it will scan.

[Voila] Looking for voila in /etc/jupyter
[Voila] Looking for voila in /usr/local/etc/jupyter
[Voila] Looking for voila in /Users/maartenbreddels/miniconda3/envs/voila/etc/jupyter
[Voila] Loaded config file: /Users/maartenbreddels/miniconda3/envs/voila/etc/jupyter/voila.json
[Voila] Looking for voila in /Users/maartenbreddels/.jupyter

TODO

• rebase on master after Allow starting voila from an instance #83 is merged
• write tests
• make tests ignore config options