feat(document): plumb in an optional file name

src/document.rs

@@ -4,6 +4,7 @@ use std::fs;
 #[derive(Default)]
 pub struct Document {
     rows: Vec<Row>,
+    pub file_name: Option<String>,
 }
 
 impl Document {
@@ -25,6 +26,9 @@ impl Document {
         for line in contents.lines() {
             rows.push(Row::from(line));
         }
-        Ok(Self { rows })
+        Ok(Self {
+            rows,
+            file_name: Some(filename.to_string()),
+        })
     }
 }