Add possibility not to reset sessions after scenario

I think it's good to reset sessions in all cases where it doesn't lead to increase of test execution time.

But in some cases it will be useful not to reset sessions after each scenario to:
1. make checks for the same page in different scenarios
2. minimize test execution time by not visitng that page before each scenario

I know that I can remove `require 'capybara/cucumber'` from `env.rb` and make tests to use own `cucumber.rb` but I think it should be possible to do it without that "monkeypatching"

I think `@no_reset_sessions` shouldn't be documented in README.md as I think this feature is dangerous and thus it should be a bit harder to find information about it.