Add Makefile and remove ad-hoc lint script

Makefile

@@ -0,0 +1,19 @@
+SHELL := /usr/local/bin/oksh
+export PATH := $(PATH)
+
+.PHONY: all fmt lint
+
+all: fmt lint test
+
+# Test
+test:
+	go test -v ./...
+
+# Format code
+fmt:
+	gofumpt -l -w .
+	gci write .
+
+# Run linter
+lint: fmt
+	golangci-lint run