Icon and Splash Screen Image Generation - Declare output directory manually

#### Short description of the problem:

I would like to choose the output directory of the icon and splash screen images myself.
#### What behavior are you expecting?

Could we expand the cli command? Something like:

```
$ ionic resources --icon --destination /my/directory/icons/
```

**Ionic Version:** 1.x

Thanks for the help and for an awesome framework.

Start with the ionic resources CLI command and trace how its options and generated image destinations are handled. Done means the command accepts a destination such as /my/directory/icons/ for icon and splash screen output, with the behavior documented or verified through the command's existing checks.

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