Requests support for multiple softsign key.

[creamjun]

Most of us use softsign key to participate in testnet.
However, because tmkms supports one softsign key,
it is inconvenient to run another tmkms process when joining a testnet on another platform.

Would you like to support mutiple softsign key for tmkms?

[tarcieri]

Yes, I'll be doing an overhaul of how keys are loaded in #388, and can look at supporting multiple softsign keys as part of that.