Add support for Scenario descriptions

[pbarnajc]

Fixes #311

Adds descriptions to Scenarios and Scenario Outlines to better fit the Gherkin syntax specifications.

[youtux]

Sorry it took so long to review it. Would you add this to the CHANGELOG.rst? I'll try to merge it and make a release next weekend.

[codecov]

Codecov Report

Base: 95.43% // Head: 95.40% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (ad6df6e) compared to base (d71bb90).
Patch coverage: 94.11% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #600      +/-   ##
==========================================
- Coverage   95.43%   95.40%   -0.03%     
==========================================
  Files          49       49              
  Lines        1751     1764      +13     
  Branches      189      193       +4     
==========================================
+ Hits         1671     1683      +12     
  Misses         53       53              
- Partials       27       28       +1     

Impacted Files	Coverage Δ
src/pytest_bdd/parser.py	98.63% <93.75%> (-0.40%)	⬇️
tests/feature/test_description.py	100.00% <100.00%> (ø)

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