Added option that allows filtering by regex.

This is helpful if you want to break generic fields into different
export items. Note that the we export everything that matches, not the
other way around.
Added plugin hooks for export.

Allows mangling the export on record level at 3 different stages:
a) before settings are applied
b) after settings are applied, i.e. the item (e.g. the XML node) is
complete
c) after the complete record is processed and the export tree is
complete