Merge pull request 'Apply Patch' (#1) from usage into master

Reviewed-on: #1

Makefile

@@ -1,11 +1,17 @@
 all:
-	go run main.go tree.go test_files/test.md
+	go build .
+
+run:
+	go run . test_files/test.md
 
 test:
-	go run main.go tree.go test_files/test.md
+	go run . test_files/test.md
-	go run main.go tree.go test_files/README.md
+	go run . test_files/README.md
-	go run main.go tree.go test_files/weird_headers.md
+	go run . test_files/weird_headers.md
-	go run main.go tree.go test_files/audio.md
+	go run . test_files/audio.md
+
+install:
+	cp -v mdtoc /usr/local/bin/.
 
 clean:
-	rm -vf main
+	go clean

main.go

@@ -61,8 +61,19 @@ func parse_file(file_name string) []heading {
 	return headings
 }
 
+func usage() {
+	fmt.Printf("Usage: mdtoc [FILE]\n")
+	os.Exit(1)
+}
+
 func main() {
-	file_name := os.Args[1]
+	var file_name string = ""
+
+	if len(os.Args) == 2 {
+		file_name = os.Args[1]
+	} else {
+		usage()
+	}
 
 	var headings []heading = nil