Automatically try alternative APIs when an API goes down

[over-engineer]

If the primary API is down, we should automatically fall back to alternative APIs.

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As @trent823 suggested in #12:

Would be good to build in automatic retry on alternative hosts in case this host also goes down

[CMiksche]

@over-engineer I created a lib for exactly that: https://github.com/CMiksche/exchangerates-wrapper