added pelican-langcategory plugin as a submodule

Pelican-langcategory plugin can make people browse articles in certain language easier. I came up this idea and found there are [other people in need of this feature](https://github.com/getpelican/pelican/issues/1393#issuecomment-53457959), therefore I implemented it.

You can see a live demo on my blog, to browse all my posts in English, [click here](http://cnborn.net/blog/lang/en/).