Handle signals in a Docker friendly way

Similar to pelias/placeholder#160, this small
change allows `ctrl+C` to immediately and gracefully shut down a docker
container started with `docker run pelias/api`.

It's a nice user experience improvement for people running our
containers on the command line.

index.js

@@ -8,3 +8,12 @@ const server = app.listen( port, host, () => {
   const listenAddress = server.address();
   logger.info( `pelias is now running on ${listenAddress.address}:${listenAddress.port}` );
 });
+
+function exitHandler() {
+  logger.info('Pelias API shutting down');
+
+  server.close();
+}
+
+process.on('SIGINT', exitHandler);
+process.on('SIGTERM', exitHandler);