Locale fallback

I created the solution to issue #55 myself. I decided not to change the behaviour of the existing function, but to add a new function called `locale`. That is probably a better name anyway, as the currency is not language-specific. The new `locale` function is similar to `language`, but more strict on the format of the given locale. In return, it offers fallback functionality.

Also, I’m sorry for the confusion, I’m quite new to Git and GitHub: This Pull Request contains three commits, but only the third (`7c354cd`) pertains to this issue, the two others are in another pull request. At least, they don’t touch the same lines.