Surround arguments with ' in config file

Hi,

Just installed `lazycli` on Ubuntu 18.04 and tried it with `ls -l`. Operations for files that contain spaces does not work with the default config file as the argument is not surrounded by '. When I add that for example `rm` works. Example:
```
- name: ls -l
registered_commands:
- ls -l
key_bindings:
- key: d
command: rm -rf '$8'
confirm: true
- key: o
command: xdg-open '$8'
- key: u
command: cd '$8'
lines_to_skip: 1
```

Thanks!