Add a rake task to check for temporary tags in project features

[Cruikshanks]

Tags such as @wip and @focus are often used when working on scenarios and features so the user can quickly just run those tests. However you don't really want them appearing in your final production code, yet they are easily forgotten.

Another project that uses Quke added a check into its TravisCI config file, but it would be nice if Quke came with a rake task to do this for you.