Make brackets optional for single attributes

This is an alternative to #483: instead of changing attributes to `@`, just make the brackets optional for `#`. 

Note that I'm not *opposed* to #483, and am only presenting this as an alternative to consider, although I have a slight preference for it. From a practical perspective, it has the benefit of being backwards compatible.

Splicing in my comment from the other RFC:

> I'd be fine with `@` as well. It's a somewhat nicer symbol... precisely *because* it's a nicer symbol, however, it has a lot of possibility as potential syntax for future language features we haven't even thought of yet, whereas `#` is most likely too ugly to use for anything else. So keeping `#` for attributes would be a more efficient use of limited ASCII real estate.

[CLICKME](https://github.com/glaebhoerl/rfcs/blob/attributes-optional-brackets/text/0000-attributes-optional-brackets.md)