Better support multiple currencies with the same format options

[dpolivy]

Creating a new issue that is related to some discussion I saw in #10 to talk about having the compiler support automatically "pre-compiling" each currencyFormatter with a pre-set list of currencies provided separately. In cases where the currency may be dynamically chosen, but from a set of known values, it would be much simpler to be able to declare the formatter once, and have the compiler automatically expand this.

A similar behavior could be added for units, which I believe has a similar pattern.

I'm not sure how to implement this, but as far as API goes, having another property on options passed to compile(), such as supportedCurrencies/supportedUnits seems like the right approach. Without this present, the compiler would take the formatters literally, but with them present (or, perhaps with them present with a special sentinel value for currency/units), it would auto-expand and generate individual formatters for each of the passed in currencies/units.