Set build output and status before after executes

I'm making altria-ikachan plugin now.

I'd like to change the message depending on the result of the build like this.
https://github.com/mizzy/altria-ikachan/blob/master/lib/altria/ikachan/notifier.rb#L18

So I'd like to set build.status attribute after job#execute_script.

Please check this pull request.

Thanks.