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.toml

@@ -38,6 +38,7 @@ ansi_term = "^0.12.1"
 ansi_colours = "^1.0"
 console = "0.14.1"
 lazy_static = { version = "1.4", optional = true }
+lazycell = "1.0"
 wild = { version = "2.0", optional = true }
 content_inspector = "0.2.4"
 encoding = "0.2"