add a version callback similar to help

[phlptp]

I am wondering how much use a dedicated version option, like help/help_all/config would be. It is similar to help in that it should bypass the other error checks and required options.

I know we end up putting it in most use cases in some way or another, having it be embedded in CLI11 would standardize and make it easier for everyone.

I would picture something like
an overload set

app.set_version(string version_string);
app.set_version(std::function<std:;string()> version_callback);  
app.set_version(string version_args, string version_string);
app.set_version(string version_args, std::function<std:;string()> version_callback);  

this would add a version option ("-v,--version" that triggers a version_exception and is triggered like help does currently.