Indicate global parameters for all subcommands

Fixes #1032 
It is necessary to add space between the section with the global parameters and the rest of the output when help is called for a subcommand like `wp post get`. This space is not necessary when `wp --help` is called.

But now that you asked I notice that this extra space is not necessary for commands like `wp post --help`. Since this PR has been merged already I should open a new one, right?