Support rules in modifier arguments

Modifiers currently support optional arguments, e.g.:

- ```#fruit.repeat(3)# ➡ appleappleapple```

But the arguments can't themselves contain tracery rules, so the following isn't permitted:

- ```#fruit.repeat(#number#)#```

This feature is currently called out in the Tracery codebase as  a "todo". It would be a useful feature for building more sophisticated modifiers.