providers/os: Add appendFile

providers/os/os.go

@@ -35,6 +35,7 @@ func (this *Provider) FuncMap () template.FuncMap {
 		"readFile":   funcReadFile,
 		"readDir":    funcReadDir,
 		"writeFile":  funcWriteFile,
+		"appendFile": funcAppendFile,
 		"renameFile": funcRenameFile,
 		"removeFile": funcRemoveFile,
 	}
@@ -99,6 +100,13 @@ func funcWriteFile (name, content string) error {
 	return err
 }
 
+func funcAppendFile (name, content string) error {
+	file, err := os.OpenFile(name, os.O_CREATE | os.O_WRONLY | os.O_APPEND, 644)
+	if err != nil { return err }
+	_, err = io.WriteString(file, content)
+	return err
+}
+
 func funcRenameFile (name, newName string) error {
 	return os.Rename(name, newName)
 }