support article category in Archive

Add category for each article and to group all articles by category instead of Year.

(also can anyone give me some advice so that I may implement this by myself. I'm currently reading relevant code and try to implement, but some advice may helps a lot.)

[imeoer]

Thanks @PoloLiu. Ink wants keep everything simple from beginning, so the article only support Tags field, and group by tag. In my opinion, the category is same with tag, it's right?