Extend YAML inventory script with _meta hostvars

Because hostvars are now returned when the `--list` option is used, the `--extra-vars` options makes no sense and is removed.
* A `--file` option is added to select the YAML inventory definition file. The default is still `hosts.yml` in the script directory.
* The `--list` option has become the default action.
* Output is sorted and indented for easy human consumption.

Because @jhoekx is the original author I would like to get his opinion about these changes too.