feature request: unapply

It would be great to have an unapply function or the ability to use this as a context manager so that one can allow nested asyncio only in special circumstances, e.g.:

```python
async def afunc():
    ...

def func():
    with nested_asyncio.apply_now() as loop:
        loop.run_until_complete(afunc)
    asyncio.get_event_loop().loop.run_until_complete(afunc) <- raises error
```