add SearchApiV2.current_page() method with tests
add SearchApiV2.per_page() method with tests
add SearchApiV2.sort() method with tests
add SearchApiV2.facets() method with tests
update docs
add SearchApiV2.category() method with tests
add SearchApiV2.topic() method with tests
add SearchApiV2.start_date() method with tests
add SearchApiV2.end_date() method with tests
add SearchApiV2.exclude_children() method with tests