Add plugin for generating tag pages automatically

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This plugin automatically generates tags after saving posts if jekyll server is running at background. The tag pages are stored in the tags/ dir, and you can add those pages to commits then push them to GitHub.

To enable this plugin, please do the following steps:

1. Create a dir named 'tags' under the root dir.
2. Create a layout, named _layouts/tag.html, for tag pages.
3. Comment out the first return statement in the plugin.
4. Update _layouts/post.html to show tag list on posts.

After enablinf this plugin, start jekyll server at local, and the server will parse all posts and generate tag pages to the tags/ dir. The tag pages are also generated on-the-fly after saving posts if jekyll server is running at background.