Add client functions to allow integrations within other CLIs.

This is a simplification of how the docker engine implements
this feature, but it will be ported there once this is merged.

Signed-off-by: David Calavera <david.calavera@gmail.com>

client/command.go

@@ -0,0 +1,37 @@
+package client
+
+import (
+	"io"
+	"os/exec"
+)
+
+// Program is an interface to execute external programs.
+type Program interface {
+	Output() ([]byte, error)
+	Input(in io.Reader)
+}
+
+// ProgramFunc is a type of function that initializes programs based on arguments.
+type ProgramFunc func(args ...string) Program
+
+// NewShellProgramFunc creates programs that are executed in a Shell.
+func NewShellProgramFunc(name string) ProgramFunc {
+	return func(args ...string) Program {
+		return &Shell{cmd: exec.Command(name, args...)}
+	}
+}
+
+// Shell invokes shell commands to talk with a remote credentials helper.
+type Shell struct {
+	cmd *exec.Cmd
+}
+
+// Output returns responses from the remote credentials helper.
+func (s *Shell) Output() ([]byte, error) {
+	return s.cmd.Output()
+}
+
+// Input sets the input to send to a remote credentials helper.
+func (s *Shell) Input(in io.Reader) {
+	s.cmd.Stdin = in
+}