Add method that directly return the adc value and get average value by digital slide filtering algorithm

[cheng3100]

Hi, I am using your code but I find that the value of vcc is sometime unstable. And sometime I just want the adc int value so that to save performance expenses. So I add a method and put a digital slide fliter inside it. I have test it in my arduino.

[cheng3100]

Well, I want to say it that my average method is almostly the same perfermance expense as get the adc value separatly. So I think maybe it is a good improvement :)
And the average method also can be apply to the Vcc calculation . I will push it if you merge the PR.