Configure Sprockets without wrapping in a "ready" block

Before, you always had to configure Sprockets like this:

```ruby
ready do
  sprockets.append_path File.join root, 'bower_components'
end
```

Now you can do:

```ruby
sprockets.append_path File.join root, 'bower_components'
```

Only certain methods are available before `ready` (`import_asset`, `append_path`, and `prepend_path`) but I'm hoping that's most of what people need.