Add optional symbol parameter to futuresPrices (#608)

* Add optional symbol parameter to futuresPrices

* Revert README.md change

Author: nonth

README.md

@@ -829,12 +829,16 @@ console.log(await client.futuresDailyStats({ symbol: 'ETHBTC' }))
 
 #### futures prices
 
-Latest price for all symbols.
+Latest price for symbol, not providing a symbol will return latest price for all symbols and is resource-expensive.
 
 ```js
 console.log(await client.futuresPrices())
 ```
 
+| Param  | Type   | Required |
+| ------ | ------ | -------- |
+| symbol | String | false    |
+
 <details>
 <summary>Output</summary>
 

index.d.ts

@@ -591,7 +591,7 @@ declare module 'binance-api-node' {
       limit?: number
     }): Promise<FuturesUserTradeResult[]>
     futuresDailyStats(options?: { symbol: string }): Promise<DailyStatsResult | DailyStatsResult[]>
-    futuresPrices(): Promise<{ [index: string]: string }>
+    futuresPrices(options?: { symbol: string }): Promise<{ [index: string]: string }>
     futuresAllBookTickers(): Promise<{ [key: string]: Ticker }>
     futuresMarkPrice(): Promise<MarkPriceResult[]>
     futuresAllForceOrders(options?: {

src/http-client.js

@@ -469,8 +469,8 @@ export default opts => {
     futuresTrades: payload =>
       checkParams('trades', payload, ['symbol']) && pubCall('/fapi/v1/trades', payload),
     futuresDailyStats: payload => pubCall('/fapi/v1/ticker/24hr', payload),
-    futuresPrices: () =>
-      pubCall('/fapi/v1/ticker/price').then(r =>
+    futuresPrices: payload =>
+      pubCall('/fapi/v1/ticker/price', payload).then(r =>
         (Array.isArray(r) ? r : [r]).reduce((out, cur) => ((out[cur.symbol] = cur.price), out), {}),
       ),
     futuresAllBookTickers: () =>