Add Gherkin (Cucumber) support

**NOTE**: This requires the [Gherkin grammar](https://atom.io/packages/cucumber) package to be installed.

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**NOTE**: This pull request piggy backs on changes I made in #95 that should be reviewed first so I can rebase this PR and make it more clear what has changed.

- [x] Update grammars.coffee
- [x] Update README.md
- [x] Add an example feature file