Consider adding memory usage to `rspec_profiling`

This is an idea from @rspeicher:

I used this listener last weekend when I was trying to debug this: https://gitlab.com/snippets/1661091

I think we could fork the rspec_profiling gem and add support for memory usage. One thing we'd need to consider is that it's not just the memory of the rspec process we're concerned about -- we also need the memory usage of the sub-process for phantomjs to see the full picture.