Add class option for default slot value

[Ambrevar]

Since 0.3.0 NIL is the default slot value.
It can be desirable to used :unbound as default, or something else.

A class option would be perfect for this.

CC @aartaka

[aartaka]

Nothing to add except: when we add it, we should default to NIL in Nyxt, but default to undefined in nclasses for the benefit/obviousness to non-Atlas libraries and apps.