Handle fractions as multipliers on big suffixes

Numerizer does not compute strings of the form:

```regex
(fraction)(of|an?|\s)+(big suffix)
```

Examples:
```text
half a million
quarter billion
```

I'm assuming that being a very narrow case, this should be fairly easy to implement. But I'm not sure if this should even be be supported as a feature depending on whether it falls into the purview of this library.