Add suport for default type parameters (TS 2.3)

**What version of TypeScript are you using?**
2.3


**What version of `typescript-eslint-parser` are you using?**
master


**What code were you trying to parse?**

```
class Foo<T = Bar> {

}
```

**What did you expect to happen?**
I expected the AST to contain the default value for type parameter T.

**What happened?**
The default type parameter value is not saved

