Support aliases for flags (#409)

Aliases are currently only supported for sub-commands, but they're
useful for flags as well. E.g., when migrating from an old flag name
to a new flag name, while still supporting the old value.

model.go

@@ -397,6 +397,7 @@ type Flag struct {
 	Xor         []string
 	PlaceHolder string
 	Envs        []string
+	Aliases     []string
 	Short       rune
 	Hidden      bool
 	Negated     bool