Improve image filter attribute handling

[Johan Janssens]

At the moment the image filter does not allow to add custom attributes to image elements. All attributes are removed and data attributes are used to provide config options on a per <img /> element basis. The problem with this approach is that its not possible to add custom data- attributes to an image using markup.

To resolve this refactor the attribute handling by the image filter as follows:

• Allow to add data- attributes and re-include them when the <img /> element is replaced with a responsive version
• Do not allow to use data- attributes to provide filter config options, instead require to the <ktml:images></ktml:images>wrapper which already does this.