Python - add decorator detection

It would be nice to recognize decorator invocation, eg:

```python
@some.decorator
def foo(bar):
    pass
```
Decorator invocation can only appear as first token on a line.

This definition should work:
```js
    'decorator': {
        pattern: /(^\s*)@\w+(\.\w+)?/i,
        lookbehind: true
    },
```
