Add a --with_frames argument to fetch

When set (defaults to not set/false), --dump will include iframe contents.

I was hoping I could add a mode to strip_mode to this, but since dump is used
extensively (e.g. innerHTML), this is something that has to be off by default
(for correctness).

src/main.zig

@@ -115,7 +115,8 @@ fn run(allocator: Allocator, main_arena: Allocator) !void {
                 .dump_mode = opts.dump_mode,
                 .dump = .{
                     .strip = opts.strip,
-                    .with_base = opts.withbase,
+                    .with_base = opts.with_base,
+                    .with_frames = opts.with_frames,
                 },
             };