Added a tag-cloud.
New features
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Added tag cloud to the layout. - 
Generates a page (for each tag) containing a list of all posts with this tag (using the file _layouts/page-tag.html)
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use site.tag_dirin_config.ymlto set the directory in which the files for each tag are created (default:tags)
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use site.tag_title_prefixin_config.ymlto set the prefix to be used for the title of the tag-pages (default:Tag:)
 
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