exchange_rate: Add precisions of some cryptocurrencies

There are many cryptocurrencies available in CoinGecko and some other
exchange rate providers. If the user wants to use a cryptocurrency as
a display currency, the precisions used to be 2. This patch adds
precisions of some cryptocurrencies.

electrum/exchange_rate.py

@@ -34,7 +34,10 @@ CCY_PRECISIONS = {'BHD': 3, 'BIF': 0, 'BYR': 0, 'CLF': 4, 'CLP': 0,
                   'JOD': 3, 'JPY': 0, 'KMF': 0, 'KRW': 0, 'KWD': 3,
                   'LYD': 3, 'MGA': 1, 'MRO': 1, 'OMR': 3, 'PYG': 0,
                   'RWF': 0, 'TND': 3, 'UGX': 0, 'UYI': 0, 'VND': 0,
-                  'VUV': 0, 'XAF': 0, 'XAU': 4, 'XOF': 0, 'XPF': 0}
+                  'VUV': 0, 'XAF': 0, 'XAU': 4, 'XOF': 0, 'XPF': 0,
+                  # Cryptocurrencies
+                  'BTC': 8, 'LTC': 8, 'XRP': 6, 'ETH': 18,
+                  }
 
 
 def to_decimal(x: Union[str, float, int, Decimal]) -> Decimal: