Fixed duplicated fields; Improved exp. taxonomies

- Changed default fields: now the default fields can be changed and are
not automatically added to the feeds.
- Added standard fields in taxonomies' selection.
- Added all selected taxonomy fields in post feeds.
Added get_terms filter

- Added filter to remove IDS' categories IDs from the names of imported
terms (regions, countries, themes) - in addition to the filter for
single terms (get_term), now there is one for multiple terms
(get_terms).