Respond to help command by showing usage info

[msakrejda]

Has gvm respond politely to requests for help.

Before:

maciek@mothra:~$ gvm help
ERROR: Unrecognized command line argument: 'help'

After:

maciek@mothra:~$ gvm help
Usage: gvm [command]

Description:
  GVM is the Go Version Manager

Commands:
  version    - print the gvm version number
  get        - gets the latest code (for debugging)
  use        - select a go version to use
  diff       - view changes to Go root
  help       - display this usage text
  implode    - completely remove gvm
  install    - install go versions
  uninstall  - uninstall go versions
  cross      - install go cross compilers
  linkthis   - link this directory into GOPATH
  list       - list installed go versions
  listall    - list available versions
  alias      - manage go version aliases
  pkgset     - manage go packages sets
  pkgenv     - edit the environment for a package set

P.S.: thanks for gvm; it's neat!