[export-html] Add new `--show` flag, remove dependency on `save-images`

[Breakthrough]

Add new --show option to display the result of export-html automatically in the browser when processing is complete. Remove requirement to explicitly specify save-images.

export-html will now always use the preceding save-images output for the images, unless --no-images is specified. If there is no explicit save-images command, it will be run with the default config values (e.g. those from the user config file).