`leftwm-theme` apply should ask users if they want to download themes that aren't already installed

When downloading and applying a theme users are required to run 

```bash
leftwm-theme install <theme-name>
leftwm-theme apply <theme-name>
```

When running apply for a theme that is not already installed it tells you that you need to go and install it. It would be nice if it asked you if you wanted it to install it for you automatically. 