copy files from wp-includes/export verbatim
copy wp-includes/iterators.php to export/iterators.php
load Oxymel via Composer
load export classes via Composer
first pass at loading and using the export API

* replace --file_item_count with --max_file_size
* --verbose is ignored, for now
add _wp_export_build_IN_condition() helper
log each exported file path

had to copy over the whole WP_Export_Split_Files_Writer class.
remove --verbose parameter

The idea is to be reasonably verbose by default and completely silent
when --quiet is passed.