Allow customization of the JAXB underlying XMLStreamWriter

Right now the jaxb component allows you to filter out non-xml characters. But there might be use cases where you'd want to do something other than just filter (maybe escape? http://blog.lesc.se/2009/03/escape-illegal-characters-with-jaxb-xml.html or replace with something else?) the non-xml characters. JAXB has a mechanism to allow pre-processing of the output of its marshaling and we should expose a way to customize that.