Modified snapshot tests to support tab expansion.

This changes how the files are named (to allow for snapshots that aren't
directly related to the --style argument) and fixes the
generate_snapshots.py script to work with the latest version of bat.

Three new tests are also introduced:
- tabs_4 - Tab expansion with a width of 4.
- tabs_8 - Tab expansion with a width of 8.
- tabs_passthrough - No tab expansion.

tests/tester.rs

@@ -38,13 +38,14 @@ impl BatTester {
         BatTester { temp_dir, exe }
     }
 
-    pub fn test_snapshot(&self, style: &str) {
+    pub fn test_snapshot(&self, name: &str, style: &str, tab_width: u32) {
         let output = Command::new(&self.exe)
             .current_dir(self.temp_dir.path())
             .args(&[
                 "sample.rs",
                 "--decorations=always",
                 "--terminal-width=80",
+                &format!("--tabs={}", tab_width),
                 &format!("--style={}", style),
             ]).output()
             .expect("bat failed");
@@ -55,7 +56,7 @@ impl BatTester {
             .replace("tests/snapshots/", "");
 
         let mut expected = String::new();
-        let mut file = File::open(format!("tests/snapshots/output/{}.snapshot.txt", style))
+        let mut file = File::open(format!("tests/snapshots/output/{}.snapshot.txt", name))
             .expect("snapshot file missing");
         file.read_to_string(&mut expected)
             .expect("could not read snapshot file");