Make Cucumber detect missing translations

Hi all.

Came across [this gist.](https://gist.github.com/oriolgual/1067007) today, and thought it might be useful here.

I've cleaned it up to try to keep as much as possible out of the global scope, but I'm not too experienced with Cucumber so if I'm diverging from the idioms feel free to point it out. I also added a few minor things like colour and outputting to STDERR. Thinking about extracting this to a Gem so it can be used in the engines also.

At the moment it doesn't fail tests for missing translations, but the functionality is there and I'd recommend enabling it.

It currently flags up one missing translation: `cy.home.show.view_all_link_expanded`