Add support for `open-settings` subcommand

From the original spec: 

> #### `open-settings`
> 
> `open-settings [--defaults,-d]`
> 
> Open the settings file. If this command is provided alone, it does not open the
> terminal window.
> 
> **Parameters**:
> * `--defaults,-d`: Open the `defaults.json` file instead of the `profiles.json`
>   file.
