Allow use of range in CLI args

Say I want to run the fourth through eighth tests. Before, I had to do:

```ruby
clash 4,5,6,7,8
```

Now, I can do:

```ruby
clash 4-8
```

The style of the code is a bit weird because I wanted to write a function, but it turns out there are no unit tests, sooooo I couldn't write a unit test for it so I just ran `bin/clash` until it all worked.

Let me know how I can better match your coding style conventions.

Good chatting with you today! :smile: