"Then show me the page" should support capybara-screenshot

[foobear]

As mentioned in #50, Then show me the page depends on a method defined by the launchy gem.

We do not want to add launchy as a dependency, but use it, if available. In addition, we use capybara-screenshot in recent projects and want the step to support its screenshot_and_save_page, too.

I recommend we make show me the page detect if either is available and use its way to create a snapshot, or fail with a helpful error message if neither is available.

Method documentation / Readme should be updated accordingly.