Output the step keyword on documentation format

Previous output:

    A simple feature
      This is a simple feature
        there is a monster -> I attack it -> it should die

Current output:

    A simple feature
      This is a simple feature
        Given there is a monster -> When I attack it -> Then it should die

[Fixes #135]
Merge pull request #139 from sobrinho/master

Output the step keyword on documentation format