Add make targets for clean and format (python-poetry#1266)

Makefile

@@ -16,6 +16,17 @@ list:
 # required for list
 no_targets__:
 
+clean:
+	@rm -rf build dist .eggs *.egg-info
+	@rm -rf .benchmarks .coverage coverage.xml htmlcov report.xml .tox
+	@find . -type d -name '.mypy_cache' -exec rm -rf {} +
+	@find . -type d -name '__pycache__' -exec rm -rf {} +
+	@find . -type d -name '*pytest_cache*' -exec rm -rf {} +
+	@find . -type f -name "*.py[co]" -exec rm -rf {} +
+
+format: clean
+	@poetry run black poetry/ tests/
+
 # install all dependencies
 setup: setup-python