Yarko/yaml config

This is a concept review branch.

Yaml config's should be easier for people to maintain.  An example config is shown in #250.

This branch works (but would need a cleanup if to be merged).   It doesn't convert json config files in existing jrnl's.  For that you can just do something like:

```
>>> import sys, os, json, yaml
>>> with open("jrnl_config", "r") as f:
...     conf = json.load(f)
... 
>>> with open("jrnl_config.yml", "w") as f:
...     yaml.dump(conf, f, indent=2, default_flow_style=False)
... 
>>> 
```


Comments appreciated.