Read options from Rails app configuration in Railtie

I wanted to disable pngout for the asset pipeline in a particular Rails env, but there wasn't a good way (from the app repo) to do this. It seemed sensible to allow settings at the app or env level so I could do something like:
```ruby
config.assets.image_optim = { pngout: false }
```

Also allows for simple case of:
```ruby
config.assets.image_optim = true
```