Allow repeat command to support range of custom numbers

Instead of having to do:
```
- repeat 10 as:value:
    - narrate <def[value].sub[5]>
```
it would be nice if all we had to do was
```
- repeat -5,5 as:value:
    - narrate <def[value]>
```

Of course, that's just a quick example. Other notation that should be considered can be `from:-5 to:5` or `-5 to 5`.