Implement --file-name<name> option

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

Closes #654

src/bin/bat/app.rs

@@ -222,6 +222,7 @@ impl App {
                     .transpose()?
                     .unwrap_or_else(|| vec![LineRange { lower: 0, upper: 0 }]),
             ),
+            filename: self.matches.value_of("file-name").or_else(|| None),
         })
     }
 

src/bin/bat/clap_app.rs

@@ -93,6 +93,18 @@ pub fn build_app(interactive_output: bool) -> ClapApp<'static, 'static> {
                      '--highlight-line 40:' highlights lines 40 to the end of the file"
                 ),
         )
+        .arg(
+            Arg::with_name("file-name")
+                .long("file-name")
+                .takes_value(true)
+                .number_of_values(1)
+                .multiple(true)
+                .value_name("name")
+                .help("Specify the name to display for a file.")
+                .long_help("Specify the name to display for a file. Useful when piping \
+                            data to bat from STDIN when bat does not otherwise know \
+                            the filename."),
+        )
         .arg(
             Arg::with_name("tabs")
                 .long("tabs")