Multiple Sections, Headers and Footers

Now there is option to add multiple headers and footers (so sections can have different headers and footers).

Also all the supported section properties are implemented according to: http://officeopenxml.com/WPsection.php

Example

    const header = this.document.createHeader();
    const footer = this.document.createFooter();

    // Add new section with another header and footer
    doc.addSection({
      headerId: header.Header.referenceId,
      footerId: footer.Footer.referenceId,
      pageNumberStart: 1,
      pageNumberFormatType: docx.PageNumberFormat.DECIMAL,
    });