Recognise exceptions as instances of Throwable

With `--level 5`, I get errors such as this one:

```
Parameter #1 $throwable of static method Foo::processThrowable() expects Throwable, FooException given.
```

Of course in PHP 7, every exception is an instance of the special `Throwable` interface.

It would be nice if PHPStan knows about it out of the box.