Allow configuration packages to be configured

[edeustace]

Allow configuration packages to be configured. This would be optional, not every element would need to have it.

An example of this would be in 'multiple-choice', allow the formatting options of the rich text editor to be customised (say only allow bold formatting).

In terms of implementation, this is mostly something that the pie-elements/* would need to add themselves and define their configuration model. For pie we'd most likely add an optional file --configuration-model (might need a rename on this front ie: question-config-file -> question-model etc). This file if present will be used to set configuration on the elements defined within it.

The file itself will look something like (for the example above):

module.exports = [
  {
    element: 'multiple-choice',
    'editable-html' : {
      activePlugins: ['bold']
    } 
  }
]