Improve output

This does two things (if `append_screenshot_path` is true):
 * The path to the screenshot is added to the RSpec metadata only if a screenshot was actually saved.
 * If a HTML file was saved, its path also gets added to the RSpec metadata.

As the path to the saved HTML file is now included in the metadata, I removed the warning in `Saver#save_html`. Less cluttered output when running the specs! :tada: