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/snapshots/sample.rs

@@ -16,3 +16,17 @@ fn main() {
 fn area(rectangle: &Rectangle) -> u32 {
     rectangle.width * rectangle.height
 }
+
+// Tab alignment:
+/*
+	Indent
+	1	2	3	4
+1	?
+22	?
+333	?
+4444	?
+55555	?
+666666	?
+7777777	?
+88888888	?
+*/