Adds user defined header to automatic help command generated.
Once a program is linked to libexarg is able to produce a help by accepting the --help or -? command line option. But that help is showed as is. This function allows you to define a header to be showed preceding the help info. You can use this, for example, to introduce your copyright.
This function doesn't make a copy of the given string so you must not free the header provided or use an static string.