Rework scenario execution to plugins

[elchupanebrej]

• scenario/scenarios could be used both as decorators and operators

[codecov]

Codecov Report

Merging #33 (b4e023a) into release_5+1 (a765488) will decrease coverage by 0.19%.
The diff coverage is 92.00%.

@@               Coverage Diff               @@
##           release_5+1      #33      +/-   ##
===============================================
- Coverage        96.08%   95.88%   -0.20%     
===============================================
  Files               54       54              
  Lines             2221     2235      +14     
  Branches           289      299      +10     
===============================================
+ Hits              2134     2143       +9     
- Misses              60       61       +1     
- Partials            27       31       +4     

Impacted Files	Coverage Δ
pytest_bdd/model/feature.py	94.59% <ø> (-2.81%)	⬇️
tests/feature/test_gherkin_terminal_reporter.py	100.00% <ø> (ø)
tests/feature/test_steps.py	100.00% <ø> (ø)
tests/feature/test_report.py	81.81% <61.53%> (-18.19%)	⬇️
pytest_bdd/scenario.py	88.97% <88.13%> (+2.30%)	⬆️
pytest_bdd/const.py	95.45% <100.00%> (+0.71%)	⬆️
pytest_bdd/generation.py	94.33% <100.00%> (+0.28%)	⬆️
pytest_bdd/hooks.py	100.00% <100.00%> (ø)
pytest_bdd/plugin.py	98.18% <100.00%> (-0.21%)	⬇️
pytest_bdd/reporting.py	91.17% <100.00%> (+1.70%)	⬆️
... and 9 more

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[elchupanebrej]

This address pytest-dev#310