Feature/strict mode

This is a duplicate of [lchenays add strict mode pull request](https://github.com/cucumber/cucumber-js/pull/204), but with fixed/added tests for the new strict flag.
I also modified the --help message for the new flag to be inline with Ruby cucumber.
The shorthand version of the flag is `-S` in cucumber-ruby. `-s` is for `--no-source`. I wish it was the other way around but I think we should not confuse people and stick to ruby's. Can you change this?