Bugfix: Save html to `html_path` instead of `file_base_name`.

I.e. honor `capybara_root` provided by the user. Without this
patch the HTML output is *always* written to the current
working directory.
Merge pull request #55 from m-o-e/fix-html_save_path

Bugfix: Save html to `html_path` instead of `file_base_name`.