bip84 for sw seeds

[SomberNight]

This enables software bip39 seeds to use p2wpkh scripts to receive on along the derivation path described in soon-to-be bip84: bitcoin/bips#620

Best not to merge until bip84 gets into draft status (above PR gets merged).
EDIT: bip84 is now in draft status

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For the time being I've disabled this (p2wpkh output scripts for receiving) for hardware wallets.

• The Trezor plugin needs some changes for it to work, but the fw might be ready. bech32 with Trezor #3551
  • also atm Trezor on master is "broken" due to merging Make Electrum work with trezorlib 0.9.0 #3621 too fast :)
• Ledger (Nano S) seems to work already with btchip-python 0.1.24, and latest fw
• KeepKey and dbb are not ready for this (though I haven't tested)
  • KeepKey needs changes in plugin, but even before that, I think python-keepkey needs some changes (and their fw too)
  • not sure about dbb but the respective plugin, again, would at least need changes

Hence, the most reasonable path forward would be to get Trezor to work ( #3551 ), and then test Trezor and Ledger a bit more, before enabling this for them. (This PR can get merged before that though.)

[coveralls]

Coverage increased (+0.08%) to 42.945% when pulling e218c4a on SomberNight:bip84 into a95534b on spesmilo:master.

[prusnak]

BIP84 has been merged into the BIPS repo: https://github.com/bitcoin/bips/blob/master/bip-0084.mediawiki