Add virtual destructors to Option and Options. Option member is set to protected mode.

include/cxxopts.hpp

@@ -1575,6 +1575,8 @@ namespace cxxopts
     {
     }
 
+    virtual ~Option() = default;
+
     std::string opts_;
     std::string desc_;
     std::shared_ptr<const Value> value_;
@@ -1656,6 +1658,8 @@ namespace cxxopts
     {
     }
 
+    virtual ~Options() = default;
+
     Options&
     positional_help(std::string help_text)
     {
@@ -1776,6 +1780,8 @@ namespace cxxopts
     void
     generate_all_groups_help(String& result) const;
 
+    protected:
+
     std::string m_program{};
     String m_help_string{};
     std::string m_custom_help{};