Enable tracing in hooks when DOKKU_TRACE is set.

This makes it much easier to debug plugins.
Also source the 'dokkurc' file when starting dokku. This makes it easy
to define DOKKU_TRACE over ssh.

plugins/nginx-vhosts/post-delete

@@ -1,3 +1,3 @@
 #!/usr/bin/env bash
-set -eo pipefail
-sudo /etc/init.d/nginx reload > /dev/null
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+sudo /etc/init.d/nginx reload > /dev/null