date and boolean data types added

lexer.h

@@ -47,6 +47,8 @@ namespace usql {
 	keyword_integer,
 	keyword_float,
 	keyword_varchar,
+	keyword_date,
+	keyword_bool,
 	keyword_distinct,
 	int_number,
 	double_number,
@@ -103,7 +105,7 @@ namespace usql {
     private:
 	TokenType type(const std::string &token);
 
-	std::string stringLiteral(std::string token);
+	static std::string stringLiteral(std::string token);
 
 	static std::string typeToString(TokenType token_type);