Save links to a database

Currently links can be saved in various formats, but they are not saved to a database.
Add a `database` format to the `--save` flag which would allow the search results to be saved to a database. Each row should contain the root URL, the time of the search and the links that were found from the search. It is up to the implementer to decide which database engine to use and how to process the data. If you have ideas for more metadata that would be useful, feel free to add it as well.