Paths of important files and directories are part of the program's
configuration, no?

We're making better use of XDG paths now.  Only configuration-y things
go in $XDG_CONFIG_HOME now, with cache-y and data-y things going in the
appropriate places instead of just being crammed in with configuration.

The configuration file is now properly overridden by argument parsing.
This was accomplished by loading the config file, then treating the
specified options as a namespace for the ArgumentParser.  This makes
things from the config file get overridden iff they were specified on
the command line (not simply from defaults set in the ArgumentParser).