Add Makefile target to run coverage

For now the target fails because there are no tests for the "cmd" package.
Since this is being used manually for now, this is acceptable.

.gitignore

@@ -1,3 +1,4 @@
 .vscode
 opa
 opalog/parser.go
+coverage

Makefile

@@ -18,7 +18,7 @@ GO := go
 GO15VENDOREXPERIMENT := 1
 export GO15VENDOREXPERIMENT
 
-.PHONY: all deps generate build test check check-fmt check-vet check-lint fmt clean
+.PHONY: all deps generate build test cover check check-fmt check-vet check-lint fmt clean
 
 all: build test check
 
@@ -36,6 +36,14 @@ build: generate
 test: generate
 	$(GO) test -v $(PACKAGES)
 
+COVER_PACKAGES=$(PACKAGES)
+$(COVER_PACKAGES):
+	@mkdir -p coverage/$(shell dirname $@)
+	go test -covermode=count -coverprofile=coverage/$(shell dirname $@)/coverage.out $@
+	go tool cover -html=coverage/$(shell dirname $@)/coverage.out
+
+cover: $(COVER_PACKAGES)
+
 check: check-fmt check-vet check-lint
 
 check-fmt: