Allow the pre-processors to be configured

[3rd-Eden]

Currenlty it's not possible to supply the pre-processor modules with any options, making it less flexible then needed. We can add an extra section to the square.json file where you can specify options that need to be passed in to the pre-processors..

eg:

{
  configuration: {
     pre-processors: {
       stylus: {
          options: 'for stylus' 
      }
    }
  }
}

See #19 for the conversation

[3rd-Eden]

Decided to go with 2 different configuration options, as you might not want to have all your stylus files to be changed to data-uri files. So instead you can now add:

{
  configuration: {},
  bundle: {
    'file.styl': {
       'pre:stylus': {
           datauri: true
       }
    }
  }
}

Where pre: is just an indication that is used for the pre-processors and stylus is the actual npm package name.