Follow GNU convention regarding usage output

[ggrossetie]

The standard --help option should output brief documentation for how to invoke the program, on standard output, then exit successfully. Other options and arguments should be ignored once this is seen, and the program should not perform its normal function.

https://www.gnu.org/prep/standards/html_node/_002d_002dhelp.html#g_t_002d_002dhelp

The following command should exit(0) and print usage to stdout:

$ asciidoctor --help

The following invalid commands should exit(1) and print usage to stderr:

$ asciidoctor file.adoc --hello

Reason: --hello is an invalid option

$ asciidoctor

Reason: The command requires at least one file