feat(config): accept callback function for method arguments

[andrenanninga]

For resizing images a certain percentage I needed access to the metadata of the image. In these changes I added the functionality to use a callback function as args for a method. This function is supplied with the metadata of an image and should return an array.

Example:

{
  method: [
    {
      name: 'resize',
      args: (metadata) => ([
        Math.round(metadata.width * 0.5),
        Math.round(metadata.height * 0.5)
      ])
    }
  ]
}

[axe312ger]

Looks good, thank you very much for your contribution :)