Made tags' and categories' path configuration more flexible.

Hi!
This PR makes it possible to configure tags and categories a bit better by:
 * allowing to set the path for categories independently of the path for tags, by introducing the configuration variable ```CATEGORY_PATH``` (which defaults to ```TAG_PATH```);
 * allowing to set the category prefix, which by default is ```cat_``` (originally used to distinguish them from tags, which traditionally had the same path);
 * allowing to determine whether to have tag and category pages as indices (instead of post lists) independently from each other by introducing ```CATEGORY_PAGES_ARE_INDEXES```;
 * and finally by adding ```CATEGORY_PAGES_DESCRIPTIONS``` which does the same as ```TAG_PAGES_DESCRIPTIONS``` but only for categories.

Comments?

If there are no categories, ```cat_items``` will be ```None```. (Same for ```items```: if there are no tags, it will be ```None```.) That's ensured in ```_create_tags_page``` (previously ```list_tags_page```).