Fix "allowMultiline" being undefined in some cases

[fatfisz]

From comment #857 (comment):

Btw. I think there was a small bug: when only "spacing" was set in the extended options, the following line was setting "allowMultiline" to undefined:

var multiline = context.options[1] ? context.options[1].allowMultiline : true;

Then in all the following conditions !multiline was being checked, which yielded a different result than the > default true value would.

Now I'll be checking if the property exists on the config object (using the has package, I found it being used in other rules).

[ljharb]

Thanks, this looks great.