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 both ```cat_items``` and ```items``` is set, it makes sense to have both ```title``` (which is "Tags and Categories") and "Categories". But if only ```cat_items``` is set (and ```items``` it not set), there's no sense to have both ```title``` ("Categories") and then another "Categories" as <h2>.