Add currency plugin for the Federal Reserve's historical data

[hsoft]

We have a bit of a problem here because the bank of canada stopped supplying historical rates before 2017, which leaves us without a rate at all for dates before that. A user who loses its currency cache could also lose the ability to properly visualize net worth data before 2017.

The Fed supplies historical exchange rates for many currencies. It's not convenient to use (we have to scrape HTML), but we don't have much of a choice here.

[hsoft]

Hum, as it turns out, BOC still has historical rates, it's just that they're separate: https://www.bankofcanada.ca/rates/exchange/legacy-noon-and-closing-rates/