Display current latest version number on GitHub when using the `--update` option if other software management solution is used on system

[Vdragon]

Please support fetching & showing the latest release when using the --update option on systems using alternative software management solution, it's useful to check whether your youtube-dl distribution is the latest.

[dstftw]

We have no control over alternative software management solution therefore they decide themselves how to update their packages.

[Vdragon]

@dstftw
Hi, I mean showing/printing only the latest upstream "version number" instead of actually updating the software, so it's not related to upgrading using alternative software management solutions.

You can check the latest version by launching a web browser and browse the Releases page but it's better if youtube-dl could show it itself.

[dstftw]

I don't think mixing official version and package version is a good idea since it can be misleading. Moreover most likely this will be patched out by package maintainers.

[yan12125]

@Vdragon FYI: The latest version number can be fetched from https://rg3.github.io/youtube-dl/update/LATEST_VERSION.

[Vdragon]

@dstftw
IMO as long as the message isn't misleading its a good idea.

@yan12125
Thanks, highly appreciated!