Add pidfile support

Copy a simple implementation from the Moby project, since importing
their package would pull in too many dependencies.

Fixes #320

docs/Webhook-Parameters.md

@@ -25,6 +25,8 @@ Usage of webhook:
         send log output to a file; implicitly enables verbose logging
   -nopanic
         do not panic if hooks cannot be loaded when webhook is not running in verbose mode
+  -pidfile string
+        create PID file at the given path
   -port int
         port the webhook should serve hooks on (default 9000)
   -secure