Addition of 'Appendix' threadmark category

[Shadowclonier]

Thanks to some idle curiosity and wishing to make accessing threadmarked information easier, we now have a new threadmark category titled "Appendix." Similarly to the preexisting index and extras categories, we're not going to be strict about how threads utilize the appendix threadmarks, it's just to give you an additional option to organize posts for greater access.

Feedback appreciated and all that.