Add ability to pass HTML or Pathname to :cover option

`wkhtmltopdf` accepts a "cover" option that you can pass in a URL or
html file into.

This change allows the `:cover` option to be specified in any of the
following ways:

    cover: 'http://example.org'

    cover: Rails.root.join('public', 'pdf', 'pdf_cover.html')

    cover: 'I AM COVERED'

    cover: '<html><head><body>I AM COVERED IN HTML</body></html>'

    cover: render_to_string('reports/cover.pdf.erb')