Added YouTube Downloader application to this repo

[TheKidPadra]

I wrote a YouTube downloader python application
I hope it's useful for this repo and gets merge
I learned a lot from this repo and I just wanted to make a contribution

[ibnaleem]

You should handle errors for unavailable itags:

1) itag="17" and res="144p"
2) itag="18" and res="360p"
3) itag="22" and res="720p"

Enter the itag of your preferred stream to download:   3

Downloading...
Traceback (most recent call last):
  File "C:\Users\xxxx\Desktop\main.py", line 59, in <module>
    ys.download()
    ^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'download'

yt.streams.get_by_itag(tag) returns None because the stream with the specified itag is not available for the particular video.