Support for Segwit XPUBs
Created by: prusnak
Accept Segwit XPUBs starting with "ypub" - see https://github.com/spesmilo/electrum/blob/0a648e2b1cdb7852d80c0626c62984ef01c1c7e9/lib/bitcoin.py#L50 for magic constants (0x049d7cb2 for segwit p2sh).
By using a different magic for Segwit XPUBs, you don't need to ask user what kind of XPUB is being scanned.
Suggestion: just detect automatically, don't ask user.