Fix: ethplorer request limit

[vanvuongngo]

Fix ETHPLORER request limit

I receive a request limit if I use the ethplorer service. The default development api key freekey has the following limits:

Requests are limited to 5 per second, 50/min, 200/hour, 2000/24hours, 3000/week.
Max age for timestamp parameter: 30 days"

Error message:

{
  "statusCode":429,
  "body": {
    "error": {
      "code":429,
      "message":"Request limit is reached for your API key"
    }
  },
  ...
}

---

I add the possibility to use a "Personal api key" which has the following limits:

Requests per second: 10
Max count of transactions/operations in response: 1000
Max age for timestamp parameter: 1 year

Register Personal a key here: https://ethplorer.io/wallet/#register

usage:

    const cb = await cryptoBalance(address, symbol, {
        ethplorerApiKey: 'YOUR_PERSONAL_API_KEY'
    });