Provide configuration parameters to data sources

Most, if not all, initialisation and/or finalisation functions will have user-configurable knobs for database connections, RESTful APIs, socket paths, or whatever other mechanism the external system the data source talks to. Rather than forcing data source implementations to use mechanisms like `System.Environment.getEnv` to access these knobs (making it painful to code *and* to use), extend retcon with a notion of *parameters*.

Parameters are parsed from a configuration file (specified in the retcon config file or by a command-line option) and the values plumbed through to the appropriate data sources as a `Map Text Text`.

Changes include:

- Add a new monad for the data source state initialise and finalise commands;

- Implement parsers for a simple configuration file format;

- Extend the CLI paraphernalia to accept a path and load the parameters file;

- Provide the parameters to data source initialisers;

- Integration tests for above.