Add support for custom completions in Go (#1035)

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>

command.go

@@ -57,6 +57,10 @@ type Command struct {
 
 	// ValidArgs is list of all valid non-flag arguments that are accepted in bash completions
 	ValidArgs []string
+	// ValidArgsFunction is an optional function that provides valid non-flag arguments for bash completion.
+	// It is a dynamic version of using ValidArgs.
+	// Only one of ValidArgs and ValidArgsFunction can be used for a command.
+	ValidArgsFunction func(cmd *Command, args []string, toComplete string) ([]string, BashCompDirective)
 
 	// Expected arguments
 	Args PositionalArgs
@@ -911,6 +915,9 @@ func (c *Command) ExecuteC() (cmd *Command, err error) {
 		args = os.Args[1:]
 	}
 
+	// initialize the hidden command to be used for bash completion
+	c.initCompleteCmd(args)
+
 	var flags []string
 	if c.TraverseChildren {
 		cmd, flags, err = c.Traverse(args)