Option to not catch SIGINT and SIGTERM

[CelticMinstrel]

These signals don't necessarily represent an error condition, as they can be sent to request the program to shut down gracefully. Thus, I think it's strange that Loguru treats them as a fatal error. Perhaps it would make sense for some programs though, so I would propose adding an additional compile-time option for it.

[jclc]

This is possible now with

	loguru::Options opts;
	opts.signal_options.sigint = false;
	loguru::init(argc, argv, opts);

So I believe this issue can be closed.