Feature request: Add timestamp columns

[petemcfarlane]

Hi, what do you think about adding timestamp columns to the database table fun_with_flags_toggles? We wanted to build some tooling to show which flags have not been used recently, to help us clean them up but we don't know when they were last used. Or do you have any other suggestions to achieve this? Similar to https://github.com/tompave/fun_with_flags/issues/173 Thanks