Parser: Add callback support for unknown "tags"

iparser.h

@@ -2,15 +2,27 @@
 #define IPARSER_H
 #include <vector>
 #include <string_view>
+#include <functional>
 #include "headline.h"
 #include "database/pagedao.h"
 #include "urlprovider.h"
 class IParser
 {
+  protected:
+	static std::string empty(std::string_view key, std::string_view content)
+	{
+		return "";
+	}
+
   public:
 	virtual std::string extractCommand(std::string cmdname, const std::string &content) const = 0;
 	virtual std::vector<Headline> extractHeadlines(const std::string &content) const = 0;
-	virtual std::string parse(const PageDao &pagedao, UrlProvider &provider, const std::string &content) const = 0;
+	virtual inline std::string parse(const PageDao &pagedao, UrlProvider &provider, const std::string &content) const
+	{
+		return parse(pagedao, provider, content, empty);
+	}
+	virtual std::string parse(const PageDao &pagedao, UrlProvider &provider, const std::string &content,
+							  const std::function<std::string(std::string_view, std::string_view)> &callback) const = 0;
 	virtual std::vector<std::string> extractCategories(const std::string &content) const = 0;
 
 	virtual ~IParser(){};