Alternative hooks syntax

I really like the idea of using strings for example definitions. In the case of hooks I think it is much better not to use strings since the possible values are limited and can be expressed more clearly as python identifiers.

My idea is to use

```python
with before.each:
    self.subject = Foo()
```

instead of 

```python
with before('each'):
    self.subject = Foo()
```

That way we also save some keystrokes :smile: