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:

I also removed the warnings for the `Rack::Test` and `Mechanize` drivers. As stated in the commit, I don’t think they are necessary and they are cluttering the output of testsuites deliberately using e.g. `Rack::Test` for some specs.