Support margin info for symbols

[vinhins]

Hi Marfusios!
Thank you for sharing the library.
Please support margin symbol info as well.
I found the document here: https://docs.bitfinex.com/reference#ws-auth-margin-info

// margin symbol calc
[
  CHAN_ID,
  TYPE, // 'miu'
  [
    'sym',
    SYMBOL,
    [
      TRADABLE_BALANCE,
      GROSS_BALANCE,
      BUY,
      SELL,
      ...
    ]
   ]
]
  
[0,"miu",["sym","tETHUSD",[149361.09689202666,149639.26293509,830.0182168075556,895.0658432466332,null,null,null,null]]]