Support relative paths for network image rendering

[erickok]

Implemented relative paths in image loading via custom image render API.

Whilst this still require modest manual work (by setting the custom image render), it solves this fairly common use case with little effort from users, and still offering much flexibility.

            // Install a custum render for all relative paths that start with the /wiki prefix
            (attr, _) => attr["src"] != null && attr["src"].startsWith("/wiki"):
                networkImageRender(baseUrl: "https://upload.wikimedia.org"),

Fixes #153 and closes #146

[tneotia]

LGTM! Simple but effective.

[ryan-berger]

Personally think this should be a wrapper around the networkImageRender. I'm open to being persuaded otherwise

[tneotia]

@erickok with this change can you also update the readme and document this property?

[erickok]

I updated the documentation too.