Added ability to set gherkin parser language via behat.yml

**Feature:** Configurable default parser language
  As a feature writer
  I want to set a default feature file language in behat.yml
  So that I do not have to write # language: xx in every file

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I am unsure how to write a test in `.feature` format for this change. Suggestions are more than welcome.

This PR is depending on https://github.com/Behat/Gherkin/pull/54