Request: on Linux, option to use system uptime instead of program uptime

In my use case at `$work`, I need to correlate loguru logs from a userspace program with dmesg output. Unfortunately, this is an old enough system that it does not support `dmesg -T` :(. Therefore, it would be very helpful for the loguru "uptime" field to have an option to use the system uptime. That can be had using [`clock_gettime()`](https://linux.die.net/man/2/clock_gettime):

```
#include
#include

int main()
{
struct timespec ts;
clock_gettime(CLOCK_BOOTTIME, &ts);
printf("%f\n", ts.tv_sec + (double)ts.tv_nsec/1e9);
return 0;
}
```

Would you be willing to add this as a compile-time or run-time option? Thank you for considering this request!