Add command to ask the relayer for its fees

[linki]

$ go run main.go orders fees \
  --exchange-contract-address=0x12459c951127e0c374ff9105dda097662a027093 \
  --maker=0x001eeaf1ec3c4aceaed088d1e7e151dd6dd0098c \
  --taker=0x0000000000000000000000000000000000000000 \
  --maker-token-address=0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2 \
  --taker-token-address=0xe41d2489571d322189246dafa5ebde1f4699f498 \
  --maker-token-amount=18981000000000000 \
  --taker-token-amount=19000000000000000000 \
  --expiration-unix-timestamp-sec=1518201120 \
  --salt=58600101225676680041453168589125977076540694791976419610199695339725548478324 \
  --relayer-url https://api.radarrelay.com/0x/v0

feeRecipient: 0xa258b39954cef5cb142fd567a46cddb31a670124
makerFee: 0
takerFee: 0