Opaque type aliases are not converted to runtime representations.

This is a:

- [x] Bug Report
- [ ] Feature Request
- [ ] Question
- [ ] Other

Which concerns:

- [ ] flow-runtime
- [x] babel-plugin-flow-runtime
- [ ] flow-runtime-validators
- [ ] flow-runtime-mobx
- [ ] flow-config-parser
- [ ] The documentation website

---
The following input results in no output from babel.
```javascript
opaque type Foo = string
```

---

### What is the expected behaviour?
It should result in the following code (maybe with a marker for opaqueness)
```javascript
const Bar = _flowRuntime2.default.type('Bar', _flowRuntime2.default.string());
```

---

### Which package versions are you using?
```
"flow-runtime": "^0.17.0",
"babel-plugin-flow-runtime": "^0.17.0"
```