Support alternate currencies from Open Exchange Rates

[tehfink]

Support for the flag "show_alternative=true" would be nice, as detailed here: https://docs.openexchangerates.org/docs/supported-currencies#alternative-black-market-and-digital-currencies

[racitup]

Do we want to do this? It would involve changing the database model to allow longer codes and descriptions because they are no longer standard 3 character, e.g. VEF_BLKMKT. This would also mean the codes are probably different across different currency sources. I'll push a separate branch but not sure this is really "production-ready"

[racitup]

You will have trouble importing alternative currencies with non-standard length codes using the --import switch using the branch above because the command code checks the code has 3 characters. Easily worked around by putting the codes into one of the supported setting variables.