Adding custom threat lists to database#48

I am integrating gglsbl as a backend for checking of URLs that are sent in our customers' messaging. This library has been extremely helpful in doing so - props for that. A question I have is whether anyone has attempted to integrate data from other sources into their database. For instance, we will be checking URLs against feeds such as [OpenPhish](https://openphish.com/), and industry-specific feeds we've gained access to. It's not super obvious from the code whether the functions for converting a URL to an entry which can be added to the database are present, though it seems like it might be possible using the function to get the hashes of a URL, plus the functions to add threat entries to the database. One area of confusion for me is how I might compute "threat prefixes". Is that just intended to be the first four hash values of a URL's hash list?