* The check option communicates default and config options to the user
as well as optional compile time features
* Move generate-completion from a subcommand to a flag in the setup
subcommand
closes#380
* --features disable_automatic_asset_installation
disables automatic installation of the `data_dir`
* a set PREFIX env will use `$PREFIX/zellij` as its `data_dir`
* since darwin doesn't have distinct data and config
directories, assumptions that we had were invalid
* now the `HOME` config directory is loaded before the
system config directory, in this case populating the
data directory doesn't invalidate possible user configuration,
because the configuration directory now exists
The split of test/no_test happens now in
find_config_dir, it always returns Null in tests.
That way differing configurations in tests shouldn't
make the test fail anymore.
