Allow Given, When and Then in feature description

[jirikuncar]

Raise only warning for following feature:

        Feature: Description

            In order to achieve something
            I want something
            Because it will be cool
            Given it is valid description
            When it starts working
            Then I will be happy


            Some description goes here.

            Scenario: Description
                Given I have a bar

[codecov]

Codecov Report

Merging #377 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #377      +/-   ##
==========================================
+ Coverage   95.51%   95.52%   +0.01%     
==========================================
  Files          48       48              
  Lines        1671     1677       +6     
  Branches      167      168       +1     
==========================================
+ Hits         1596     1602       +6     
  Misses         43       43              
  Partials       32       32              

Impacted Files	Coverage Δ
pytest_bdd/__init__.py	100.00% <100.00%> (ø)
pytest_bdd/exceptions.py	100.00% <100.00%> (ø)
pytest_bdd/feature.py	99.23% <100.00%> (+0.01%)	⬆️
tests/feature/test_description.py	100.00% <100.00%> (ø)

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[jirikuncar]

This can be resolved by using official parser (see #378).

[jirikuncar]

@olegpidsadnyi can you please have a look at this patch? It would help until the official parser is used. Thanks!