Configuration files that override global defaults

You can now create a handful of YAML configuration files, whose contents
will override any precompiled default values.  The following files are
checked, with earlier files taking precedence over later files:

  $XDG_CONFIG_HOME/buzzy.yaml
    (which is usually $HOME/.config/buzzy.yaml)
  $XDG_CONFIG_DIRS/buzzy.yaml
    (which is usually /etc/xdg/buzzy.yaml)
  $XDG_DATA_HOME/buzzy/config.yaml
    (which is usually $HOME/.local/buzzy/config.yaml)
  $XDG_DATA_DIRS/buzzy/config.yaml
    (which is usually $PREFIX/share/buzzy/config.yaml)

The final one is intended for default values that would be cumbersome
to compile in to the buzzy executable.  We don't have any of those yet,
so that file doesn't exist.  But if we want to use this mechanism to
define default values, we'll just need to create a new config.yaml file
and have the cmake build scripts install it into $PREFIX/share/buzzy.
Empty $PREFIX/share/buzzy/config.yaml

Just to get it out of the way, I've updated the cmake build scripts to
install an empty $PREFIX/share/buzzy/config.yaml file.  That way, if we
do decide to use it to define any default values in the future, we just
have to edit the empty file.
Merge branch 'feature/multi-config' into develop

* feature/multi-config:
  Empty $PREFIX/share/buzzy/config.yaml
  Configuration files that override global defaults