Bittrex createOrder: unify 'stopPrice'
Bittrex createOrder: unify 'stopPrice'

Added stop order functionality to Bittrex:
```
node examples/js/cli bittrex createOrder BTC/USDT LIMIT BUY 0.0002 30000 '{"stop":"true","operand":"LTE","triggerPrice":"0.1","orderToCreate":"true"}'

bittrex.createOrder (BTC/USDT, LIMIT, BUY, 0.0002, 30000, [object Object])
2022-04-19T21:36:23.892Z iteration 0 passed in 1676 ms

{
  id: '423da3f7-eb03-4a8e-ac9c-011ff402a4c4',
  clientOrderId: undefined,
  timestamp: 1650404184330,
  datetime: '2022-04-19T21:36:24.330Z',
  lastTradeTimestamp: 1650404184330,
  symbol: 'BTC/USDT',
  type: 'limit',
  timeInForce: 'GTC',
  postOnly: false,
  side: 'buy',
  price: 30000,
  stopPrice: '0.1',
  cost: undefined,
  average: undefined,
  amount: 0.0002,
  filled: undefined,
  remaining: undefined,
  status: 'open',
  fee: { cost: undefined, currency: 'USDT' },
  info: {
    id: '423da3f7-eb03-4a8e-ac9c-011ff402a4c4',
    marketSymbol: 'BTC-USDT',
    operand: 'LTE',
    triggerPrice: '0.1',
    orderToCreate: {
      marketSymbol: 'BTC-USDT',
      direction: 'BUY',
      type: 'LIMIT',
      quantity: '0.0002',
      limit: '30000',
      timeInForce: 'GOOD_TIL_CANCELLED'
    },
    status: 'OPEN',
    createdAt: '2022-04-19T21:36:24.33Z',
    updatedAt: '2022-04-19T21:36:24.33Z'
  },
  trades: [],
  fees: [ { cost: undefined, currency: 'USDT' } ]
}
```

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