Re-implement raw CSS imports for the deprecation warning

Raw CSS imports have been removed from LibSass. We need to opt into
the feature with `sass_option_push_import_extension`. We're must do
this to maintain BC. Any imported files with a `.css` extension will
produce a deprecation warning.

This commit should be reverted in v5. This would remove raw CSS
imports once and for all.

src/binding.cpp

@@ -113,6 +113,7 @@ int ExtractOptions(v8::Local<v8::Object> options, void* cptr, sass_context_wrapp
   sass_option_set_precision(sass_options, Nan::To<int32_t>(Nan::Get(options, Nan::New("precision").ToLocalChecked()).ToLocalChecked()).FromJust());
   sass_option_set_indent(sass_options, ctx_w->indent);
   sass_option_set_linefeed(sass_options, ctx_w->linefeed);
+  sass_option_push_import_extension(sass_options, ".css");
 
   v8::Local<v8::Value> importer_callback = Nan::Get(options, Nan::New("importer").ToLocalChecked()).ToLocalChecked();