Add split-key address mining support

[Limeth]

https://en.bitcoin.it/wiki/Split-key_vanity_address
https://bitcointalk.org/index.php?topic=81865.msg901491#msg901491

[10gic]

@Limeth There is a similar project that support split-key address, see https://github.com/10gic/vanitygen-plusplus#split-key-vanity-address-generation