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 recommend using the standard practice for everything, even if something
else would be responsible for rebuilds.

-- 
Chris Warrick <https://chriswarrick.com/>
Sent from my Galaxy S3.
On Dec 21, 2014 1:31 PM, "felixfontein" <notifications@github.com> wrote:

> In nikola/plugins/task/tags.py
> <https://github.com/getnikola/nikola/pull/1535#discussion-diff-22147415>:
>
> >
> >          if not self.site.posts_per_tag and not self.site.posts_per_category:
> >              return
> >
> > +        if self.site.config['CATEGORY_PATH'] == self.site.config['TAG_PATH']:
>
> 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.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/getnikola/nikola/pull/1535/files#r22147415>.
>