Display copyright, as-is notice and license declaration with `help` and `version` sub-commands

[smoyer64]

As recommended in the GPLv3 at https://github.com/MichaelMure/git-bug/blob/896d7cb1b03d449f399bd835693188c3910866a5/LICENSE#L655.

Note that these should also be shown in termui and webui when the appropriate functions are executed.

[MichaelMure]

I'm not against it if that's necessary but I'd also like to keep the legal burden to a minimum. Is that helping really? What for?

I see a majority of OSS project around without this and it doesn't seem to be a problem. Is that a case of having belt plus suspender (as we say in french)?

[smoyer64]

IANAL - Both Firefox and Chromium have ways to view dependencies and their associated licenses through the "About" menu item. I've definitely seen it elsewhere but it's often very hidden. It is a requirement when using the GPLv3 - I don't know what happens if you don't do it.