Allow specific currencies

[SlexAxton]

https://github.com/format-message/message-format-rb/blob/master/lib/message_format/interpreter.rb#L91

Currently, using a message like Your total is: {amount, number, currency} can only support USD (Since that's the default in Twitter's CLDR lib[0]).

I'm unsure of the best way to add this functionality (I'm no ruby pro), but it'd be quite helpful.

Until then, it's decently easy to just pass in a Twitter CLDR formatted currency as a string, but it'd be cool to do all in one spot!

Thanks!

[0] https://github.com/twitter/twitter-cldr-rb/blob/4054e3ee23c42716d14c38231c7b39169a3afad4/lib/twitter_cldr/formatters/numbers/currency_formatter.rb#L11

[vanwagonet]

I'm no ruby pro either. :) I added custom formats to the related JavaScript library a while back, so I can certainly take a look at making them work here as well.

I've got a bunch of competing priorities right now, though, so if you'd like to try to implement it in a pull request, you can probably get it sooner.

[SlexAxton]

I'll see what I can do. I think I'd classify this as a bug, since trying to format EUR or CAD currencies through the currency number helper will output incorrect values, but it probably doesn't matter what the label is.