feat(config): Inheritance

Add support for basic inheritance. The parser will
look for `inherit = base/section` and copy all undefined
values from the base section.

Ref #84

include/components/config.hpp

@@ -24,9 +24,12 @@ DEFINE_ERROR(key_error);
 
 class config {
  public:
+  static constexpr const char* KEY_INHERIT{"inherit"};
+
   explicit config(const logger& logger, const xresource_manager& xrm) : m_logger(logger), m_xrm(xrm) {}
 
   void load(string file, string barname);
+  void copy_inherited();
   string filepath() const;
   string bar_section() const;
   vector<string> defined_bars() const;