Add `SimpleLogger::with_monotonic_timestamps`

[newpavlov]

We use simple_logger as logger for our tests. We also run MIRI for some of our tests. Unfortunately, MIRI intentionally does not emulate SystemClock with enabled isolation, so we either have to disable isolation, or use SimpleLogger::without_timestamps. The latter works, but we lose information about timings.

It would be nice if simple_logger had support for timestamps based on std::time::Instant. It could remember Instant during initialization and use Instant::elapsed for log entry timestamps.

[0xrodrigues]

I would like to work on this issue. As a beginner in the Rust language, I believe this project — and this issue in particular — could be a great opportunity for me to improve my Rust skills while contributing. Let me know if there are any guidelines I should follow before getting started.

[borntyping]

It should be fine to work on this, though do bear in mind the notice from the README - I try to only merge small improvements or fixes that meet my aim keeping the project very simple. I think this feature probably fits that, especially since there's already a mechanism for different types of timestamp and any information they need (the Timestamps enum).