Add ability to mark flags as required or exclusive as a group (#1654)

This change adds two features for dealing with flags:
 - requiring flags be provided as a group (or not at all)
 - requiring flags be mutually exclusive of each other

By utilizing the flag annotations we can mark which flag groups
a flag is a part of and during the parsing process we track which
ones we have seen or not.

A flag may be a part of multiple groups. The list of flags and the
type of group (required together or exclusive) make it a unique group.

Signed-off-by: John Schnake <jschnake@vmware.com>

command.go

@@ -863,6 +863,10 @@ func (c *Command) execute(a []string) (err error) {
 	if err := c.validateRequiredFlags(); err != nil {
 		return err
 	}
+	if err := c.validateFlagGroups(); err != nil {
+		return err
+	}
+
 	if c.RunE != nil {
 		if err := c.RunE(c, argWoFlags); err != nil {
 			return err