Allow rule decorators to be used without args

I've seen people with this issue a couple times, most recently: https://github.com/venmo/business-rules/issues/16

This PR will allow variable decorators to be used with or without params/parens, like so:

```python
@string_rule_variable(label="My variable!")
def my_var(self):
    return "foo"

### OR

@string_rule_variable
def my_var(self):
    return "foo"
```