special help subcmd prints to stdout; small improvements for args/flag errors

[zacharysyoung]

Addresses #83.

Print special help subcommand to to stdout for better readability/processing: since the user explicitly asks for help, make their life easier by printing to stdout.

bad/missing subcommand error, and subcommand -h usage still print to stderr.

Also, following the style of something like go test -h, which prints:

usage: go test [build/test flags] [packages] [build/test flags & test binary flags]
Run 'go help test' and 'go help testflag' for details.

A missing subcommand prints:

usage: gocsv help|version|<subcommand>
Run 'gocsv help' for a list of subcommands, and 'gocsv <subcommand> -h' for subcommand details.

and a bad subcommand (e.g., foo), prints:

error: invalid subcommand "foo"

usage: gocsv help|version|<subcommand>
Run 'gocsv help' for a list of subcommands, and 'gocsv <subcommand> -h' for subcommand details.

A bad/missing subcommand no longer prints the big usage list, which will likely obscure the error/problem.