Add support for Scenario descriptions

[pbarnajc]

Fixes https://github.com/pytest-dev/pytest-bdd/issues/311

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

[codecov[bot]]

Codecov Report

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

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

```diff @@ 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](https://codecov.io/gh/pytest-dev/pytest-bdd/pull/600?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytest-dev) | Coverage Δ | | |---|---|---| | [src/pytest\_bdd/parser.py](https://codecov.io/gh/pytest-dev/pytest-bdd/pull/600?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytest-dev#diff-c3JjL3B5dGVzdF9iZGQvcGFyc2VyLnB5) | `98.63% <93.75%> (-0.40%)` | :arrow_down: | | [tests/feature/test\_description.py](https://codecov.io/gh/pytest-dev/pytest-bdd/pull/600?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytest-dev#diff-dGVzdHMvZmVhdHVyZS90ZXN0X2Rlc2NyaXB0aW9uLnB5) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytest-dev). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytest-dev)

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