This commit allows programs using Cobra to code their custom completions
in Go instead of Bash.
The new ValidArgsFunction field is added for commands, similarly to
ValidArgs. For flags, the new function
Command.RegisterFlagCompletionFunc() is added.
When either of the above functions is used, the bash completion script
will call the new hidden command '__complete', passing it all
command-line arguments. The '__complete' command will call
the function specified by Command.ValidArgsFunction or by
Command.RegisterFlagCompletionFunc to obtain completions from the
program itself.
Signed-off-by: Marc Khouzam <marc.khouzam@montreal.ca>
