Request should be more flexible

[Piotr Jasiun]

It should be easy to add some additional parameters to the request. Parameters defined in the upload widget definition should be passed to the fileLoader, to the fileUploadRequest and finally to the request build by the default fileUploadRequest handler. So for example:

fileTools.addUploadWidget( editor, 'sepiauploadimage', {
    additionalRequestParameters: { sepia: true },

    // ...
} );

Will add evt.data.additionalRequestParameters.sepia = true to the fileUploadRequest and send it to the server (formData.append( 'sepia', 'true');).