parser: Add [content] tag, add extractFirstTag() method

parser.h

@@ -9,6 +9,7 @@ class Parser : public IParser
 	std::string processImage(std::smatch &match) const;
 
   public:
+	std::string extractFirstTag(std::string tagname, const std::string &content) const override;
 	std::string extractCommand(std::string cmdname, const std::string &content) const override;
 	std::vector<std::string> extractCommands(std::string cmdname, const std::string &content) const override;
 	std::vector<Headline> extractHeadlines(const std::string &content) const override;