Add symbol parameter to futures price ticker

[klemensz]

The "Symbol Price Ticker" of the Futures API accepts a symbol parameter: https://binance-docs.github.io/apidocs/futures/en/#symbol-price-ticker

It should be added to the "futures prices" function: https://github.com/Ashlar/binance-api-node#futures-prices

That way it will be possible to get the price for just one symbol, which has a lower weight than getting it for all symbols.

[bgarciaoliveira]

up

[balthazar]

PRs welcome

[nonth]

@balthazar I've created PR #608 for this issue, hope it can help