add blockstream.info as explorer option (#4829)

electrum/util.py

@@ -625,6 +625,8 @@ mainnet_block_explorers = {
                         {'tx': 'tx/', 'addr': 'address/'}),
     'Blockr.io': ('https://btc.blockr.io/',
                         {'tx': 'tx/info/', 'addr': 'address/info/'}),
+    'Blockstream.info': ('https://blockstream.info/',
+                        {'tx': 'tx/', 'addr': 'address/'}),
     'Blocktrail.com': ('https://www.blocktrail.com/BTC/',
                         {'tx': 'tx/', 'addr': 'address/'}),
     'BTC.com': ('https://chain.btc.com/',
@@ -656,6 +658,8 @@ testnet_block_explorers = {
                        {'tx': 'tx/', 'addr': 'address/'}),
     'Blockchain.info': ('https://testnet.blockchain.info/',
                        {'tx': 'tx/', 'addr': 'address/'}),
+    'Blockstream.info': ('https://blockstream.info/testnet/',
+                        {'tx': 'tx/', 'addr': 'address/'}),
     'BTC.com': ('https://tchain.btc.com/',
                        {'tx': '', 'addr': ''}),
     'smartbit.com.au': ('https://testnet.smartbit.com.au/',