add Makefile with linters (black & isort)

- borrowed makefile self-help code from SecureDrop
- considered windows dev env case: GNU make available via Cygwin

Makefile

@@ -0,0 +1,37 @@
+.PHONY: lint-black
+lint-black: ## check python source code formatting issues, with black
+	black --check --diff ./
+
+.PHONY: lint-black-apply
+lint-black-apply: ## apply black's source code formatting suggestions
+	black ./
+
+.PHONY: lint-isort
+lint-isort: ## check imports are organized, with isort
+	isort --check-only ./
+
+.PHONY: lint-isort-apply
+lint-isort-apply: ## apply isort's imports organization suggestions
+	isort ./
+
+.PHONY: lint
+lint: lint-black lint-isort ## check the code with various linters
+
+.PHONY: lint-apply
+lint-apply: lint-black-apply lint-isort-apply ## apply all the linter's suggestions
+
+# Makefile self-help borrowed from the securedrop-client project
+# Explaination of the below shell command should it ever break.
+# 1. Set the field separator to ": ##" and any make targets that might appear between : and ##
+# 2. Use sed-like syntax to remove the make targets
+# 3. Format the split fields into $$1) the target name (in blue) and $$2) the target descrption
+# 4. Pass this file as an arg to awk
+# 5. Sort it alphabetically
+# 6. Format columns with colon as delimiter.
+.PHONY: help
+help: ## Print this message and exit.
+	@printf "Makefile for developing and testing dangerzone.\n"
+	@printf "Subcommands:\n\n"
+	@awk 'BEGIN {FS = ":.*?## "} /^[0-9a-zA-Z_-]+:.*?## / {printf "\033[36m%s\033[0m : %s\n", $$1, $$2}' $(MAKEFILE_LIST) \
+		| sort \
+		| column -s ':' -t