Add option to remove blank lines between function docstring and code

I'd like to suppress blank lines between a function's docstring and its code:

```
def foo(x, y):
"""
Do awesome stuff.
"""

return x + y
```

should be formatted as

```
def foo(x, y):
"""
Do awesome stuff.
"""
return x + y
```

Perhaps an option like `BLANK_LINE_AFTER_FUNCTION_DOCSTRING` (cf. #716).