Handle fractions as multipliers on big suffixes

[dhruvkb]

Numerizer does not compute strings of the form:

(fraction)(of|an?|\s)+(big suffix)

Examples:

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.

[jduff]

If it doesn't complicate the codebase too much I would be happy to accept a PR for this.