subscribe (channel, payload) {
    this.send(Object.assign({
      event: 'subscribe',
      channel
    }, payload))
  }
Subscribe to a channel with the given filter payload

@param {string} channel - channel payload/data
@param {object} payload - optional extra packet data
@example
const ws = new WSv2()

ws.on('open', () => {
  ws.onTrades({ symbol: 'tBTCUSD' }, (trades) => {
    // ...
  })

  ws.subscribe('trades', { symbol: 'tBTCUSD' })
})

await ws.open()