Remote manifest items support, part 1

[vers-one]

Remote manifest items support, part 1

This is:

• a bug fix
• an enhancement

Related issue: #61

Description

This pull request adds support for remote manifest items (i.e. files referenced by absolute URLs like http://example.com/book/123/chapter1.html as opposed to local files like Content/chapter1.html which are packaged within the EPUB file).

Testing steps

Try to open a EPUB book with the following manifest items:

<manifest>
  <item id="item-1" href="https://example.com/books/123/test.html" media-type="application/xhtml+xml" />
  <item id="item-2" href="https://example.com/books/123/image.jpg" media-type="image/jpeg" />
  ...
</manifest>