Allow the library to be filtered by tags.

This add an argument to `listBooksIds` to filter by tags.
So, this is an API break.

include/library.h

@@ -194,6 +194,7 @@ class Library
     const std::string& language = "",
     const std::string& creator = "",
     const std::string& publisher = "",
+    const std::vector<std::string>& tags = {},
     size_t maxSize = 0);
 
   friend class OPDSDumper;