Allow rule decorators to be used without args

[gregeinfrank]

I've seen people with this issue a couple times, most recently: #16

This PR will allow variable decorators to be used with or without params/parens, like so:

@string_rule_variable(label="My variable!")
def my_var(self):
    return "foo"

### OR

@string_rule_variable
def my_var(self):
    return "foo"

[gregeinfrank]

@akurihara

[fuhrysteve]

👍 Using this and it is working great for me so far!

[akurihara]

@manda986 and I looked at this, looks good!