hack way to fix 'mark' bug which cannot handle subtask well

append spaces to subtasks, which will not be displayed,
but different length of trailing spaces means belonging to
different upper level tasks.

meanwhile benefits purge cmd as well

src/taskbox.rs

@@ -78,6 +78,9 @@ impl TaskBox {
         let mut tasks = Vec::new();
         let mut title = String::new();
 
+        let mut postfix_sub = String::new();
+        let mut last_is_sub = false;
+
         for (index, rline) in fs::read_to_string(&self.fpath)
                             .expect("Failed to read file")
                             .lines().enumerate() {
@@ -86,19 +89,28 @@ impl TaskBox {
             if index == 0 {
                 title = line.trim_start_matches("# ").to_string();
 
-            } else if let Some(stripped) = line.strip_prefix(PREFIX) {
-                tasks.push((stripped.to_string(), false))
-            } else if let Some(stripped) = line.strip_prefix(PREFIX_DONE) {
-                tasks.push((stripped.to_string(), true))
+            } else if line.starts_with("- [") {
+                if let Some(stripped) = line.strip_prefix(PREFIX) {
+                    tasks.push((stripped.to_string(), false))
+                } else if let Some(stripped) = line.strip_prefix(PREFIX_DONE) {
+                    tasks.push((stripped.to_string(), true))
+                } else { continue }
+
+                if last_is_sub {
+                    last_is_sub = false;
+                    postfix_sub += " "; // hack way to identify sub-tasks belong to diff task
+                }
             } else {
                 // might be sub-tasks
                 let line = line.trim_start();
 
                 if let Some(stripped) = line.strip_prefix(PREFIX) {
-                    tasks.push((SUB_PREFIX.to_owned() + stripped, false))
+                    tasks.push((SUB_PREFIX.to_owned() + stripped + &postfix_sub, false))
                 } else if let Some(stripped) = line.strip_prefix(PREFIX_DONE) {
-                    tasks.push((SUB_PREFIX.to_owned() + stripped, true))
-                }
+                    tasks.push((SUB_PREFIX.to_owned() + stripped + &postfix_sub, true))
+                } else { continue }
+
+                last_is_sub = true;
             }
         }
 
@@ -112,7 +124,7 @@ impl TaskBox {
         for (mut task, done) in self.tasks.clone() {
             if let Some(left) = task.strip_prefix(SUB_PREFIX) {
                 content.push_str("  ");
-                task = left.to_owned();
+                task = left.trim_end().to_string();
             }
 
             if done {