I'd like a mechanism to optionally avoid registering a custom function if it has already been registered.

SQLite doesn't seem to offer a way to introspect registered custom functions so I'll need to track what has already been registered in sqlite-utils instead.

Should I register the custom rank_bm25 SQLite function for every connection, or should I register it against the connection just the first time the user attempts an FTS4 search? I think I'd rather register it only if it is needed.

Originally posted by @simonw in #198 (comment)