Support quadrillions and quintillions

[nfriedly]

JS's max safe integer is about 9 quadrillion, but there's no reason to stop there when we're approximating anyways.

Since both quadrillion and quintillion start with a "q", we'd want to choose something longer like "quad" and "quint". And maybe have an option to spell out the word.

See also:

• https://en.wikipedia.org/wiki/Names_of_large_numbers
• https://stackoverflow.com/q/25502521/933879