flag: add support for sub-commands

[gopherbot]

by raul.san@sent.com:

When you have so much flags is better to separate it with sub-commands, being the first
flag such sub-command, just like is used in go command source code:
"http://golang.org/src/cmd/go/main.go"; using the Command struct.

It is ver useful so should be in the standard library; if the flag's library API want
not be changed, it could be added in flag/flagutil

[bradfitz]

Comment 1:

Subcommands can be built using the flag package as-is.  Camlistore and other projects do
this already.
Let's not add more stuff to the flag package which can be done already easily.

Status changed to WorkingAsIntended.

[gopherbot]

Comment 2 by raul.san@sent.com:

If there is a standard way to handle it, then could be built a tool to read the command
source where one want to generate the completion-data for a shell like bash or zsh.
https://groups.google.com/d/msg/golang-nuts/_5cc9SdVOS4/9exVlbM8D50J