Add Support for Cryptocurrency API Route

[prikhi]

Need some way to specify the cryptomarket & which currencies should hit this route instead of the stock route.

https://www.alphavantage.co/documentation/#currency-daily

[the-solipsist]

Alphavantage provides two lists CSV files of of the currencies they handle, which I've used thusly in a bash script for myself:

curl -s https://www.alphavantage.co/physical_currency_list/ | \
  awk -F',' '{print $1}' > /tmp/currency-list

curl -s https://www.alphavantage.co/digital_currency_list/ | \
  awk -F',' '{print $1}' >> /tmp/currency-list

hledger -f $all_file -f $foreign_file stats | \
  grep -E '^Commodities' | grep -w -o -f /tmp/currency-list > /tmp/valid-currencies

readarray -t foreign_currencies < /tmp/valid-currencies

So, you could similarly checking commodity names and aliases against those two lists.

[prikhi]

Latest master has cryptocurrency support but it requires manual specification w/ cli flags or configuration options(see README or --help).

Eventually it'd be cool to use that digital_currency_list CSV that alphavantage provides. Maybe something like:

1. fetch CSV, save in $XDG_CACHE_DIR
2. write current time to $XDG_CACHE_DIR, use to bust cached CSV file daily/weekly
3. In fetchPrices, fetch/read CSV file & use to split commodities into equites & cryptos.

I'll leave this open until I implement that...