Create zampy configuration file + management

[BSchilperoort]

For the user's download, ingest, etc. directories we want to set these in a config file. For example:
~/.config/zampy/config.json

This is also required to be able to run recipes using the command line: the only thing required is a valid configuration file and the info in the recipe.

This config file will also need to hold user's keys for datasets that require it, for example CDS and ADS.

Originally posted by @BSchilperoort in #11 (comment)