Add cover tasks to test code coverage

Makefile

@@ -40,6 +40,14 @@ updatedeps:
 		| sort -u \
 		| xargs go get -f -u -v
 
+cover:
+	@go tool cover 2>/dev/null; if [ $$? -eq 3 ]; then \
+		go get -u golang.org/x/tools/cmd/cover; \
+	fi
+	go test $(TEST) -coverprofile=coverage.out
+	go tool cover -html=coverage.out
+	rm coverage.out
+
 # vet runs the Go source code static analysis tool `vet` to find
 # any common errors.
 vet: