Add option to enable timestamps in logging

[benbjohnson]

Litestream doesn't currently use timestamps in its log messages because timestamps are usually attached by the supervisor (e.g. systemd). Adding a flag to enable timestamps would be helpful for some folks though.

Discussed in https://github.com/benbjohnson/litestream/discussions/256

^{Originally posted by insinger December 22, 2021} Have you considered including a time/datestamp on any of the messages sent to the console? I've had a case where I can see several lines with messages like "sync error: create generation: initialize shadow wal: read header: EOF", a situation which was eventually resolved by actions taken by the application. But from looking at the console messages I can't tell whether that situation is still active, or when it was resolved. In other words, was the last message written 1 minute ago or 1 day ago? A timestamp would really help in this case.

Similarly, it would be nice to at-a-glance see when the snapshot was written when I see a message "snapshot written 7c04f098aa5bc5b2/00000000". I know I could use the CLI 'snapshots' command to figure this out, but having it right there alongside the message is mighty convenient.