Reduce startup time in loop-through mode with 80%-90%

Instead of 100 ms - 50 ms, startup takes 10 ms - 5 ms.

HighlightingAssets::get_syntax_set() is never called when e.g. piping the bat
output to a file (see Config::loop_through), so by loading the SyntaxSet only
when needed, we radically improve startup time when it is not needed.

Cargo.lock

@@ -101,6 +101,7 @@ dependencies = [
  "globset",
  "grep-cli",
  "lazy_static",
+ "lazycell",
  "nix",
  "path_abs",
  "predicates",