add xml.format="long" option to read/write a date field as a long value

before Modello 1.0, XPP3 plugin was generating reader/writer code with long as default format for dates (other plugins were using other default formats)

MODELLO-126 unified default date format to yyyy-MM-dd'T'HH:mm:ss.SSS, which is the more accurate date format compatible with XML Schema
then it is impossible with Modello 1.0 to read/write a date as a long, since custom format with xml.format define a SimpleDateFormat pattern, which does not provide such option

idea: xml.format="long" option value, which won't be considered as a SimpleDateFormat pattern but as a special case to treat the date as long