Show all subcommands summary in rbenv-help

before:
```text
$ rbenv
rbenv 0.4.0-82-g783618b
Usage: rbenv <command> [<args>]

Some useful rbenv commands are:
   commands    List all available rbenv commands
   local       Set or show the local application-specific Ruby version
   global      Set or show the global Ruby version
   shell       Set or show the shell-specific Ruby version
   install     Install a Ruby version using ruby-build
   uninstall   Uninstall a specific Ruby version
   rehash      Rehash rbenv shims (run this after installing
executables)
   version     Show the current Ruby version and its origin
   versions    List all Ruby versions available to rbenv
   which       Display the full path to an executable
   whence      List all Ruby versions that contain the given executable

See `rbenv help <command>' for information on a specific command.
For full documentation, see: https://github.com/sstephenson/rbenv#readme
```

after:
```text
$ bin/rbenv
rbenv 0.4.0-82-g783618b
Usage: rbenv <command> [<args>]

Some useful rbenv commands are:
   --version   Display the version of rbenv
   commands    List all available rbenv commands
   exec        Run an executable with the selected Ruby version
   global      Set or show the global Ruby version
   help        Display help for a command
   hooks       List hook scripts for a given rbenv command
   init        Configure the shell environment for rbenv
   install     Install a Ruby version using ruby-build
   local       Set or show the local application-specific Ruby version
   prefix      Display prefix for a Ruby version
   rehash      Rehash rbenv shims (run this after installing
executables)
   root        Display the root directory where versions and shims are
kept
   shell       Set or show the shell-specific Ruby version
   shims       List existing rbenv shims
   uninstall   Uninstall a specific Ruby version
   version     Show the current Ruby version and its origin
   version-file   Detect the file that sets the current rbenv version
   version-name   Show the current Ruby version
   version-origin   Explain how the current Ruby version is set
   versions    List all Ruby versions available to rbenv
   whence      List all Ruby versions that contain the given executable
   which       Display the full path to an executable

See `rbenv help <command>' for information on a specific command.
For full documentation, see: https://github.com/sstephenson/rbenv#readme
```