Allow file:///path/to/html uri in url list

[derklempner]

Description
Currently urls are fetched from http://some/domain/here specified in the url list, but it would be useful to be able to fetch files from disk (also for use in local cache #41 )

Proposed solution
Check for file URI and load appropriately. Potentially two routes for the config, 1) files on disk could appear in the urls list section just as file:///path/to/file.html, or 2) perhaps have a new paths or files list (maybe the logical separation is useful?).