[FR] Add support for custom placeholder transforms in config

[zizther]

It would be good to be able to control the parameters of the placeholder image, so that it can be controlled via the config file. This would include parameters such as height, width, quality, blur ... all elements really so that the output could be anything required per project.

In my case I use Imgix as the transformer, it would be good to be able to use some of the features here as well.

This has been partially mentioned in FR #208