Add support for a configuration file

It would be helpful to have a configuration file to insert headers, api_url, and so on automatically at each call.

Start by tracing how the command-line tool currently handles headers and api_url for each call, then identify the request setup entry point and related tests. Done means a configuration file can supply these values automatically on every call, with tests covering the supported configuration behavior.

Written by the indexing model from the issue text.