chifra export --holdings

[tjayrush]

Add a command that:

1. runs statements on every tx in an account's history
2. prints only the list of uniq tokens that appear in assetAddr

Super simple. Super useful because then user can run

for each account {
    for each month {
       for each holding {
          print $currency balance of holding for account at end of month
       }
    }
}