Phemex reduceMargin, addMargin
Phemex reduceMargin, addMargin

Added reduceMargin and addMargin to Phemex:

reduceMargin:
```
phemex.reduceMargin (BTC/USD:BTC, -0.0001)
2022-05-10T09:30:14.363Z iteration 0 passed in 555 ms

{
  info: { code: '0', msg: '', data: 'OK' },
  type: 'reduce',
  amount: -0.0001,
  code: 'BTC',
  symbol: 'BTC/USD:BTC',
  status: 'ok'
}
```

addMargin:
```
phemex.addMargin (BTC/USD:BTC, 0.0001)
2022-05-10T09:28:05.407Z iteration 0 passed in 580 ms

{
  info: { code: '0', msg: '', data: 'OK' },
  type: 'add',
  amount: 0.0001,
  code: 'BTC',
  symbol: 'BTC/USD:BTC',
  status: 'ok'
}
```

ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt