Visual Editing should use custom HTML processor for a field when configured

It should be possible to use a custom HTML processor in a field instead of the default one. For example:

1. Copy the node /hippo:configuration/hippo:modules/htmlprocessor/hippo:moduleconfig/richtext to /hippo:configuration/hippo:modules/htmlprocessor/hippo:moduleconfig/customprocessor
2. Change the allowed elements and attributes under the 'customprocessor' node (e.g. remove one of the allowed elements)
3. Use the document type editor to set the htmlprocessor.id property of a rich text field to "customprocessor"
4. When saving HTML in that field in the Visual Editor, the backend should use the customprocessor and hence remove the no-longer-allowed elements from the HTML.