For a single root command with a Run method, the help output still
contains 'help [command]' as a subcommand (because Help is always
added). Since the only subcommand would be 'help', the help is better
off omitted.
This change allows a command to be used both as a subcommand
or a root command without having to define a custom help that elides
the help command when no subcommands are added. Instead, the default
help command is only added when subcommands are present.
If, for some reason, you have an application with some name "foo", and your
app has a subcommand "foo", cobra should behave properly when you call
"foo foo", and it should also behave if you call "foo f".
These changes verify both of these cases and ensure cobra responds properly.
A command can now be invoked with a prefix of its own name, assuming that
prefix is unambiguous (ie it isn't also a prefix of any sibling command's
name).
