Better TypeScript support

[SharonGrossman]

Hi, first of all thanks for the great package. I would like to point that I've had some issues with the package when using TypeScript, as the data array is not actually typed - codes() returns a string[] when it's supposed to be something like CurrencyCode[], because it is in fact a closed set of values (currency codes).

It led me to write this sort of a fork to this package: https://github.com/osskit/currencies

But I would really want to keep using this package instead - just if we could better granular types for the codes, return values of the getters, etc. I'm not sure what's the maintaining status of this package, so I'll wait for a response, and in the mean time use the above suggested package. Thanks in advance!