Implement a command to fund a Testnet account from Oasis Testnet Faucet

[anzoman]

It would be useful to have a command that would (re)fill your account with TEST tokens, so the users don't have to visit the Faucet page.

Similarly, for ROFL apps running on Testnet, we could reuse this functionality and ask the user if they want to refill if running out of funds.

Something like: oasis account faucet-drop or similar.