Add a custom path for pruning

[lokki007]

You use "Screenshot.capybara_root" all the time, but in my project screenshots are not in the root of my tests folder.

"capybara_root" method sets capybara_tmp_path = Capybara.save_and_open_page_path || '.', which is either makes me using deprecated "save_and_open_page_path" method or set it it '.'

Is there any chance you can add a support for a custom location I can provide instead of '.' ?

[mattheworiordan]

@lokki007 sure, feel free to create a PR if you need this functionality.