title: Add Custom Url to WP All Import with function editor
body: <p>Hi guys and thanks for the help.
  I have a remote server that implements an OAI-PMH interface to collect data from a database.
  I want import those records using wp all import through 'download from url'.
  Since i can query the oai interface with a url that can contain customs parameters (because i don't want the entire collection), it is possible to use the function editor of wp_allimport to change or attach (dynamic) parameters to the url before the import is executed? And then schedule the same? </p>