Per steer from project core member, add other valid coinmarketcap

listed crypto base currencies that are valid during conversion lookup

Here is the test of USDT working:
https://api.coinmarketcap.com/v2/ticker/1027/?convert=USDT&limit=10

CMK page lists: "BTC", "ETH" "XRP", "LTC", and "BCH" as valid.

freqtrade/constants.py

@@ -29,7 +29,7 @@ SUPPORTED_FIAT = [
     "EUR", "GBP", "HKD", "HUF", "IDR", "ILS", "INR", "JPY",
     "KRW", "MXN", "MYR", "NOK", "NZD", "PHP", "PKR", "PLN",
     "RUB", "SEK", "SGD", "THB", "TRY", "TWD", "ZAR", "USD",
-    "USDT"
+    "BTC", "ETH", "XRP", "LTC", "BCH", "USDT"
     ]
 
 # Required json-schema for user specified config