Add folding support for multi-line decorators

This change allows decorator arguments to span multiple lines without creating two folds, e.g.

```python
@decorator_with_args(long_names, or_many, arguments,
                     2)
def foo():
    print 'foo'
```

folds to:
```python
def foo(): --------------------------------------------------------------------
```

instead of:
```python
                     2) -------------------------------------------------------
def foo(): --------------------------------------------------------------------
```