Extract author info from wordpress XML files

Extract author info from wordpress XML files. Upon running the `wordpressdotcom` importer, it will first parse all `wp:author` nodes in the XML which contain the all the authors' metadata, and add it at the header of each file, for example:

    ---
    layout: page
    title: Example
    categories: []
    tags: []
    status: publish
    type: page
    published: true
    meta:
      _edit_last: '1'
      _wp_page_template: example-page.php
    author:
      login: jdoe
      email: jdoe@example.com
      display_name: John Doe
      first_name: John
      last_name: Doe
    ---