Pretty print for Cipher txes

[matevz]

Currently, data field is Base64-encoded when showing:

$ oasis contracts instantiate 286 '{instantiate: {initial_counter: 43}}' --paratime cipher
Unlock your account.
? Passphrase: 
You are about to sign the following transaction:
Format: plain
Method: contracts.Instantiate
Body:
  {
    "code_id": 286,
    "upgrades_policy": {
      "address": "oasis1qrpaguksger8dqq8pn0vcnu8kr4m29ssvqk7kn0y"
    },
    "data": "oWtpbnN0YW50aWF0ZaFvaW5pdGlhbF9jb3VudGVyGCs=",
    "tokens": []
  }
Authorized signer(s):
  1. MJ2XCjkj132C9YWpDUSQFjkCTI8bSw8bi0w9EwwE1Bg= (ed25519)
     Nonce: 3
Fee:
  Amount: 0.001417535 TEST
  Gas limit: 283507
  (gas price: 0.000000005 TEST per gas unit)

Network:  testnet
ParaTime: cipher
Account:  test

Decode it and show it as JSON.