[Suggestion] Add theme categories

**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](https://github.com/hexojs/site/blob/master/source/_data/themes.yml) 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](https://github.com/hexojs/site/blob/master/source/_data/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](https://hexo.io/themes/)
8. 🎉