Simplify adding htracker to XDG_DATA_HOME path.

src/main.rs

@@ -74,7 +74,7 @@ fn main()
 	let mut hmgr = HabitMgr::new();
 
 	let data_dir:PathBuf = match env::var("XDG_DATA_HOME") {
-		Ok(s) => [ s, "htracker".to_string() ].iter().collect(),
+		Ok(s) => PathBuf::from(s).join("htracker"),
 		Err(_e) => PathBuf::from("./"),
 	};