Add script to convert between old and new style Hush addresses

Example:

$ ./contrib/convert_address.py t1QkTnFb9hsvkfn1kaEVf2c6pzQhkCM9zvD
RGA3rS4JLNwj99L46yfezKL7W1yG5MiUam

contrib/convert_address.py

@@ -0,0 +1,29 @@
+#!/usr/bin/env python3
+# Copyright (c) 2019 Hush developers
+
+import sys
+import base58
+from binascii import unhexlify
+from base58 import b58encode, b58decode_check
+from hashlib import sha256
+
+# based on https://github.com/KMDLabs/pos64staker/blob/master/stakerlib.py#L89
+def addr_convert(prefix, address, prefix_bytes):
+    rmd160_dict  = {}
+    # ZEC/HUSH/etc have 2 prefix bytes, BTC/KMD only have 1
+    # NOTE: any changes to this code should be verified against https://dexstats.info/addressconverter.php
+    ripemd       = b58decode_check(address).hex()[2*prefix_bytes:]
+    net_byte     = prefix + ripemd
+    bina         = unhexlify(net_byte)
+    sha256a      = sha256(bina).hexdigest()
+    binb         = unhexlify(sha256a)
+    sha256b      = sha256(binb).hexdigest()
+    final        = b58encode(unhexlify(net_byte + sha256b[:8]))
+    return(final.decode())
+
+if len(sys.argv) < 2:
+    sys.exit('Usage: %s hushv2address' % sys.argv[0])
+
+address = sys.argv[1]
+# convert given address to a KMD address
+print(addr_convert('3c', address,2))