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?

I thought these two settings should not go into ```kw```, since they are only used to compute the output file names in ```tag_*_path/link``` and ```category_*_path/link```, since if the file name changes, everything (well, at least all tag/category pages and indices) will be rebuilt anyway.