Add a way not to include all locales

[yamadapc]

There should be a means of not including all locales.

A very simple solution would be to move the logic to lib/core and keep lib/index including all locales, so people would be able to manually require just the logic and the locales they'll use:

import writtenNumber from 'written-number/lib/core';
import writtenNumberEn from 'written-number/lib/i18n/en';
// ...

[leoseccia]

+1 from me!

[mtkcs]

I made a PR #70 that exposes a standalone version that does not add all the locales. No breaking changes were introduced, please take the time to review it.