Fix numbers that start with a

Hi Roman! Here's another one. :) This PR includes the tests from my [other PR](https://github.com/bolgovr/numerizerJS/pull/3) and a few other changes. I'll close the previous PR.

- Fixes the bug described in https://github.com/bolgovr/numerizerJS/pull/3
- Alternate implementation for the fix for https://github.com/bolgovr/numerizerJS/issues/1

All tests pass.