Allow empty values in the confifiguration file.

This allows 'exclude-plugins=' to be used.
Only show the filename if present.
Perform environmental-variable expansion in the config file.

This allows the following to work:

        database = $HOME/foo.db
Moved configuration-file reader into a module.

Rather than reading the configuration file in the main chronicle
script we now delegate that to a module. This allows testing too,
whcih is a win.
Perltidy