fix references to powers of ten

[markburns]

see https://github.com/markburns/numbers_in_words/pull/6/files#r46365729

[dimidd]

Right, it should be language dependent, e.g. some languages have a word for 10000, while English doesn't. How do I access the language inside the NumberParser module?

[markburns]

Actually, I think we should wait until we have one more language before thinking about this: once #12 is done it will be clearer which code belongs where. Otherwise we're guessing too much upfront. (Which is what I had already done when initially writing this stuff anyway).