Improve config definitions and defaults

[pnadolny13]

We extract configs like encoding and others through the code (e.g. encoding default definitions) and cast to the defaults. It would be better to do the default coalescing at the top most level for consistency, to allow --about to return those defaults, and for better readability.

Also we define the files array in meltano.yml without the rich type definitions. We should update that to include the full output of --about --format=json.