usage: fix formatCmdUsage

[balupton]

This fixes the behaviour of formatCmdUsage

• before: it outputs the apps flags and arguments
• after: it outputs the commands flags and arguments

Template file to test:

{{define "FormatCommands"}}\
{{range .FlattenedCommands}}
## {{formatCmdUsage $ .}}
{{template "FormatUsage" .}}\
{{end}}\
{{end}}\
\
{{define "FormatUsage"}}
{{if .Help}}\
{{.Help|Wrap 0}}\
{{end}}\
{{end}}\
\
{{if not .Context.SelectedCommand}}\
# <command> [<args> ...]
{{template "FormatUsage" .App}}
{{end}}

{{if .Context.Flags}}
Flags:
{{.Context.Flags|FlagsToTwoColumns|FormatTwoColumns}}
{{end}}

{{if .Context.Args}}
Args:
{{.Context.Args|ArgsToTwoColumns|FormatTwoColumns}}
{{end}}

{{if .Context.SelectedCommand}}
{{if len .Context.SelectedCommand.Commands}}
Subcommands:
{{template "FormatCommands" .Context.SelectedCommand}}
{{end}}\
{{else if .App.Commands}}
Commands:
{{template "FormatCommands" .App}}
{{end}}