Specify LoaderContext with allowCodeImport = true

Using the SWF library on AIR 4.0.0 for Windows, I was receiving the following error:

```
SecurityError: Error #3226: Cannot import a SWF file when LoaderContext.allowCodeImport is false.
	at flash.display::Loader/_loadBytes()
	at flash.display::Loader/loadBytes()
	at format.swf::SWFLibrary/load()[C:\HaxeToolkit\haxe\lib\swf\git\format\swf\SWFLibrary.hx:123]
	at openfl::Assets$/loadLibrary()[C:\HaxeToolkit\haxe\lib\openfl\2,0,0\openfl\Assets.hx:876]
	at Preloader/onLoaded()[C:\Users\Ben\Dropbox\Projects\Snowman\source\Preloader.hx:63]
	at ApplicationMain$/current_onEnter()[C:\Users\Ben\Dropbox\Projects\Snowman\export\flash\haxe\ApplicationMain.hx:128]
```

This pull request fixes this by specifying a LoaderContext with allowCodeImport = true.