Add command to cancel a stuck transaction

[fernandomg]

[fvictorio]

I would like to add --nonce and --gas-price flags to tx:nop. That way, the cancellation would mean doing just eth tx:nop --pk mypk --nonce <same-nonce-as-tx-to-cancel> --gas-price <triple-of-tx-gas-price>. Once we have this, we can have a eth tx:cancel --pk mypk <tx-hash> as a shortcut to that.