Add $limit to get_post_ids() escape_ngg_limit query arg

Set &escape_ngg_limit=12 to limit the migration to a specific number, useful for cases where you want to test it out first, or need to split up the processing for really really large sites without access to WP-CLI and restricted timeouts (WPEngine for example).
Merge pull request #9 from sc0ttkclark/patch-2

Add $limit to get_post_ids() escape_ngg_limit query arg