Change `setFormat` to accept `Maybe DateFormat`.
Commit 5cf41025 introduced a bug which prevented client code from being able to clear the date format in settings. By implicitly wrapping the argument in a `Just` the actual timestamp rendering kicked in which produced an empty field in the log output. This commit fixes this behaviour by changing `setFormat` to take a `Maybe DateFormat` parameter. Client code can pass in `Nothing` to disable timestamps.