When using 2 or more config files, the values of the second would
improperly overwrite nested dictionaries of the first one.
The new method properly combines these nested dictionaries as well.
- remove support for old windows config paths
- catch exception if cache-database can't be opened
- fix username/password settings for unit tests
- rename variable 'max_tries' to 'retries'
- speed improvements, especially in the 'interpolate' function
- 'interpolate' now prioritizes base-level values if they exist
- "username" is chosen before "extractor.<category>.username"
- -u/--username & co can now override config-file values
