Add a command for funding an account on testnet

[chasefleming]

Having to manually add your address to fund an account from the Faucet site while developing is a little tedious. It would be nice if you could run a command like the following to fund an account:

flow fund testnet-acct

This would take the account from your flow.json and send it 1000 like on the Faucet site. We could also add a --amount flag if the dev wanted to customize the amount but it could also be added later. Not sure the flag is necessary.

Note: should error with a message if you try to fund an account that isn't testnet.

[sideninja]

I like the fact we would have a group of common interactions made simple during development (emulator, testnet). This also falls under similar category than #1286
A new developer should have a very easy way to do these actions with 0 configuration.

[ianthpun]

@bthaile mind if i take this?