Gherkin (Cucumber feature definition) mode

Here's another mode :smile_cat:

actually got a freelance commission to write full feature test coverage for an existing app, so I'll be writing lots of Gherkin the next few weeks… Ace's built-in mode works quite nicely, just had to enable it.

Is there any reason why we shouldn't just enabled all modes that ship with Ace? (Yes I'm volunteering to do the work if you say it's a good idea)