Add a Makefile for easy testing many version

The `Makefile` includes `stack test` configurations to support building
versions of `brittany` with supported versions of `ghc`. Each version
uses a separate `.stack-work` directory to allow minimal compilation on
each change.

.gitignore

@@ -13,3 +13,4 @@ cabal.sandbox.config
 cabal.project.local
 .ghc.environment.*
 result
+.stack-work*

Makefile

@@ -0,0 +1,28 @@
+.PHONY: test
+test:
+	echo "test"
+	stack test
+
+.PHONY: test-all
+test-all:
+	$(MAKE) test test-8.6.5 test-8.4.3 test-8.2.2 test-8.0.2
+
+.PHONY: test-8.6.5
+test-8.6.5:
+	echo "test 8.6.5"
+	stack test --stack-yaml stack-8.6.5.yaml --work-dir .stack-work-8.6.5
+
+.PHONY: test-8.4.3
+test-8.4.3:
+	echo "test 8.4.3"
+	stack test --stack-yaml stack-8.4.3.yaml --work-dir .stack-work-8.4.3
+
+.PHONY: test-8.2.2
+test-8.2.2:
+	echo "test 8.2.2"
+	stack test --stack-yaml stack-8.2.2.yaml --work-dir .stack-work-8.2.2
+
+.PHONY: test-8.0.2
+test-8.0.2:
+	echo "test 8.0.2"
+	stack test --stack-yaml stack-8.0.2.yaml --work-dir .stack-work-8.0.2