Setup signal to trap SIGINT and gracefully stop server

This way, when you send `SIGINT` to a running a server, when running via
`rails s`, a graceful shutdown/stop will be performed.

Before this, not trapping `SIGINT` would mean that either `Rails::Server#start`
or `::Rack::Server#start` would be trapping `SIGINT` and exiting the process.

lib/puma/launcher.rb

@@ -398,6 +398,14 @@ def setup_signals
         log "*** SIGTERM not implemented, signal based gracefully stopping unavailable!"
       end
 
+      begin
+        Signal.trap "SIGINT" do
+          stop
+        end
+      rescue Exception
+        log "*** SIGINT not implemented, signal based gracefully stopping unavailable!"
+      end
+
       begin
         Signal.trap "SIGHUP" do
           if @runner.redirected_io?