Shut down gracefully for signals

[BenasB]

Hi 👋

This PR adds crude SIGTERM and SIGINT signal handling to call server.Shutdown and allow the program to successfully shutdown with the 0 exit code.

closes #109

Testing

I manually tested the following scenarios after running process-exporter and:

1. Calling pkill -15 process-exporte
2. Calling pkill -2 process-exporte
3. Pressing Ctrl + C

and then checking the exit code with echo $? which should be 0

[ncabatoff]

Thanks @BenasB !