Added the option to display downloaded images using the prefix 'file:'

[edurossi]

Hello, I added to the loading of images the option to load from downloaded files as I need on my project. I think the change is very little and maybe someone else would need it too.
I read that I should open an issue first and talk about it, but as the change is very small and I already did it for my project, I just want to share it with you and you can decide if you want it or not.
Thank you for this project, I really like it !

[erickok]

Let's discuss: #497

[tneotia]

Hi there, thanks for this PR! With #505, we have finally added this capability into the library with a custom image rendering API (much better than the current customRender). You can see the discussion at #497 to see how it works. If you have any suggestions, please let @erickok know there. Again thanks for contributing! :)

[erickok]

Duplicate of #478 and see there for more discussion, but basically you can now do this:

  // Some path to where your images are
  final localRoot = "/app/accessible/path/to/images";

  // Handle all paths that start with a / and treat as local
    (attr, _) => attr["src"] != null && attr["src"].startsWith("/wiki"):
        (context, attr, element) {
      final file = File(localRoot + attr["src"]);
      return Image.file(file);
    },