[Suggestion] Add theme categories

[DeepSpaceHarbor]

Summary At the moment there are over 265 hexo themes. Finding the right theme can be quite the challenge.

Proposal

1. Create list of all tags used in current themes.
2. Extract the tags that could be used as category. Ex: Responsive, Portfolio, Blog, etc...
3. Themes that don't belong in any category should be classified as Other.
4. Create new field categories in themes.yml. This will contain the category data.
5. Use the results from step 2 and 3 to automatically update the current themes.
6. If needed, add more categories to the list.
7. Add category selection/sidebar to the themes section
8. 🎉

[RammusXu]

Proposal:

• order by github star, github last commit

It's really help to let people know if a theme is still maintaining.

[stevenjoezhang]

See also https://github.com/hexojs/site/issues/837