Add a signal handler to flush logs on exit

[alanz]

At the moment if HIE is killed for any reason, it discards any logs that have not been flushed.

Add a set of signal handlers to flush the logs on SIGTERM etc.

Possible resource: http://zguide.zeromq.org/hs:interrupt

https://hackage.haskell.org/package/logging-3.0.2/docs/Control-Logging.html
https://hackage.haskell.org/package/fast-logger-2.4.1/docs/System-Log-FastLogger.html

Note: there is nothing holy about this logging package, it was just convenient.

[DanielG]

How about flushing every line in the log instead? Signal handlers are probably a pain most platforms other than Linux (i.e. WinDOS).

[gracjan]

Line flushing is better. In case of kill -9 nothing is lost.

[alanz]

This should probably go away as part of #112

[alanz]

We flush on every write to the log now, from #120