Add —skip_ir option to process minimum IR code generation

--skip_ir option basically does minimum IR code generation and returns whole code instead of generating native code for individual AST nodes.

```bash
bin/hyperloop compile --platform=ios --log-level=trace --src=../hyperloop-ios/examples/basic --arch=i386 --skip_ir
```

generated code:

```
// ---- IR code generation ----

// sourcecode:1
auto var10 = JSStringCreateWithUTF8CString(...);
auto var11 = JSStringCreateWithUTF8CString("app.js");
JSEvaluateScript(ctx,var10,object,var11,1,exception);
JSStringRelease(var10);
JSStringRelease(var11);
CHECK_EXCEPTION(exception);
```