support new plugin 'pelican_tag_cloud_by_category'

[Plugin](https://github.com/znotdead/pelican_tag_cloud_by_category.git)
Changes are made to display only tags that belongs to certain category (on category pages).

Example:
You have 2 categories: 
 1. recipes
 2. modelling

Home page has tag cloud:
* soup
* drink
* sandwich
* car
* plane

Recipes page has tag cloud: 
* soup
* drink
* sandwich

Modelling page has tag cloud:
* car
* plane

To use it you should install plugin and setup pelicanconf.py:
```
DISPLAY_TAGS_FOR_CATEGORY = True
```