Allow customization of signal handler in registration, add flushing

[nextsilicon-itay-bookstein]

Thank you for making this library, it's very useful to us!

This PR tries to address a couple of minor pain points we encountered when using the library:

• Makes signal handler installation logic reusable and allows for caller to specify an alternative handler (Linux/Darwin)
• Adds flushing of the output streams from crash handlers

😄

[bombela]

Thank you for offering your work.

The signal handler that comes with backward-cpp is an example more than something ready for all use-cases. Consider it as a basic example ready for copy paste to make your own.

With my experience maintaining this library over the year, I have observed that implementation of signal handlers can change widely depending of the application (sync, async, threading etc). Therefore I think it is better to keep this outside of backward-cpp.