Add union type support

- Add union type support
- Add —import option to specify import headers
- Include GLKit and OpenGLES framework by default

Because not all framework has `FRAMEWORK_NAME/FRAMEWORK_NAME.h` headers, we need something to deal with default headers to import. This PR adds `--import` option to add headers.

```
hyperloop compile --imports="OpenGLES/EAGL" --imports="GLKit/GLKit" --src=...
```

will be expanded into:

```
#import OpenGLES/EAGL.h
#import GLKit/GLKit.h
```