Monad w/ config for source init/finalise functions

Add a new monad to read configuration details in the data source
initialise and finalise operations. The next step is to actually feed
some data through.
Simple configuration file format for sources

Add a parser and some massaging functions to handle a very simple file
format for data source configuration details. Data produced will be
passed to initialisers in the new monad.
Add parameters file to retcon configuration
Sample parameters file for testing
Basic tests for parameter parsing
Merge branch 'master' into wip-datasource-config
Use new initialisers in storage tests
Clean up some storage tests
Tests for parameter passing
Option parser now loads params from specific file

The helper function to build a 'RetconOptions' from system settings
file and command-line options now loads the data sources parameters
from an additional file, if the option is supplied.

These additional parameters will be supplied to data source
initialisers, making it easy to supply authentication, connection, and
other run-time information to them.
Plumb params to data source initialisers
Pin version of optparse-applicative
End-to-end tests of data source parameter handling

Update the configuration-test suite to cover the plumbing of
parameter values through to data source initialisers and finalisers.
Merge pull request #26 from anchor/wip-datasource-config

Provide configuration parameters to data sources