Add HIVE to SLIP-44

This should be coordinated with those with experience working with key derivations from seed phrases on things like the Ledger wallet. Currently STEEM is listed, but HIVE is not. Once an agreed to hexa is selected (maybe the next open one: 509 | 0x800001fd?) it should be as simple as submitting a pull request. Comments on other PS say to use the first available number.

https://github.com/satoshilabs/slips/blob/master/slip-0044.md