Change storage setting from storage.yml to heroku config.

Hi, there.
Since I want to create my blog by publify on heroku, I modified some files in publify for my heroku app.
But, I notice this changes cause merge conflict when I get new commits from master. I have to resolve conflicts manually at each time. I think it's not smart. 
Then, I think it's one of good way to use [heroku config](https://devcenter.heroku.com/articles/config-vars).

Even if you don't feel good, you could change from storage.yml to storage.yml.example for avoiding conflicts.