Added Support for using jest mock functions

[seanlion]

Overview

A workaround for returning values to use jest mock functions.
I think it would be helpful for users who wants to integrate with jest.

Usage

var redis_client = require("redis-mock")
jest.fn().mockImplementation(async (args) => {
    const result = await new Promise((resolve, reject) => {
        redis_client.command(args, function(err,result)
         {
            resolve(result);
         });
    });
    return result;
});