Add users (#258)

For #248 

Added a User DB similar to the Device DB. 
The user specification be added with the API/mdmctl commands and then referenced with a blueprint. 
If added to the blueprint, the user will be installed as an administrator during the AccountConfiguration step.

cmd/mdmctl/get_users.go

@@ -0,0 +1,47 @@
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"os"
+	"text/tabwriter"
+
+	"github.com/pkg/errors"
+
+	"github.com/micromdm/micromdm/core/list"
+)
+
+type usersTableOutput struct{ w *tabwriter.Writer }
+
+func (out *usersTableOutput) BasicHeader() {
+	fmt.Fprintf(out.w, "UUID\tUDID\tUserID\tUserShortName\tUserLongName\n")
+}
+
+func (out *usersTableOutput) BasicFooter() {
+	out.w.Flush()
+}
+
+func (cmd *getCommand) getUsers(args []string) error {
+	flagset := flag.NewFlagSet("users", flag.ExitOnError)
+	flagset.Usage = usageFor(flagset, "mdmctl get users [flags]")
+	if err := flagset.Parse(args); err != nil {
+		return err
+	}
+
+	w := tabwriter.NewWriter(os.Stderr, 0, 4, 2, ' ', 0)
+	out := &usersTableOutput{w}
+	out.BasicHeader()
+	defer out.BasicFooter()
+
+	users, err := cmd.list.ListUsers(context.TODO(), list.ListUsersOption{})
+	if err != nil {
+		return errors.Wrap(err, "list users")
+	}
+
+	for _, u := range users {
+		fmt.Fprintf(out.w, "%s\t%s\t%s\t%v\t%s\n", u.UUID, u.UDID, u.UserID, u.UserShortname, u.UserLongname)
+	}
+
+	return nil
+}