Add support for Rule keyword (Gherkin 6)

[Zayon]

Since Gherkin version 6 a new keyword "Rule" has been added to the Gherkin syntax.

See the Gherkin Reference for usage examples of the Rule keyword

Feature: Highlander

  Rule: There can be only One

    Scenario: Only One -- More than one alive
      Given there are 3 ninjas
      And there are more than one ninja alive
      When 2 ninjas meet, they will fight
      Then one ninja dies (but not me)
      And there is one ninja less alive

    Scenario: Only One -- One alive
      Given there is only 1 ninja alive
      Then he (or she) will live forever ;-)

  Rule: There can be Two (in some cases)

    Scenario: Two -- Dead and Reborn as Phoenix
      Given ....

Related to #1404

[ciaranmcnulty]

Yep, #1404 addresses this but progress has stalled somewhat

[Zayon]

Can I help somehow ?