Improve warning detection in output

[roberts1000]

Some projects use the rspec_starter gem to further control how rspec is started. The rspec_starter gem can run other commands that execute before rspec starts and those commands many trigger warning messages. rspec_n currently isn't catching those warning messages so it doesn't show a yellow (Warnings) flag in the results table. They're visible in the individual rspec_n_itermation.* files so it should be possible to expand the logic that's searching for warnings. The same problem would likely occur when the -c option is used to create a custom compound command. If one of the commands, that runs before rspec starts, throws a warning, rspec_n would likely miss it.