Implement pip2 help command

[njwilson]

Implement a 'help' command to display usage information for the other commands. It should be usable as, for example, pip2 help install and pip2 install --help

[rlbaker]

pip2 now displays help in the current situations:

• pip2 (without args)
• pip2 install --help (or any supported command)
• pip2 --help

pip2 help install is not currently implemented due to argparse adding any custom commands to the usage list which would create duplicates in the usage list.
Example:

usage: pip2 [-h] {install,uninstall,freeze,search,help} ...

positional arguments:
  {install,uninstall,freeze,search,help}

optional arguments:
  -h, --help            show this help message and exit