Implement --file-name<name> option

- can specify filename to be displayed when printing.
- useful for when piping data from STDIN

Closes #654

src/config.rs

@@ -70,6 +70,9 @@ pub struct Config<'a> {
 
     /// Ranges of lines which should be highlighted with a special background color
     pub highlighted_lines: HighlightedLineRanges,
+
+    /// Name of file to display when printing
+    pub filename: Option<&'a str>,
 }
 
 #[test]