- Support df command

- Return warnings array from ParseSyntaxFiles() in addition to potential
  error. Not being able to parse the dir is a fatal error but if a
random file cannot be read, it should be considered a warning.

examples/syncat.go

@@ -28,11 +28,14 @@ func main() {
 	}
 
 	var defs []*highlight.Def
-	err := highlight.ParseSyntaxFiles(syn_dir, &defs)
+	err, warnings := highlight.ParseSyntaxFiles(syn_dir, &defs)
 	if err != nil {
 		log.Fatal(err)
 	}
 
+	// it's up to you what to do with the warnings. You can print them or ignore them
+	fmt.Println(warnings)
+
 	highlight.ResolveIncludes(defs)
 
 	fileSrc, _ := ioutil.ReadFile(os.Args[1])