Usage output should default to stdout not stderr for subcommands too

[rhcarvalho]

#71 fixed #31, making command --help | grep something work as expected, however it doesn't work for subcommands like command subcmd --help | grep something.

This can be reproduced by compiling the example program from the README, and:

$ go run main.go echo --help | grep help        # won't work
$ go run main.go echo --help 2>&1 | grep help   # hack

[n10v]

I can't reproduce it, It works for me

[rhcarvalho]

Thanks to git bisect, I identified it was fixed back in #303.