Adds Goodreads plugin, git ignore rules and fixes latex module

I have added a plugin that can be used to display user's Goodreads activity. It's properly documented.

Project didn't have any ignore rules defined. I have added git ignore rules for Python, Vim, Emacs, Windows, Linux and OSX. I think it cover majority of the userbase so this will be useful for other users too.

Lastly, I added `__init__.py` to Latex and Goodreads plugin. Without it, it is hard to figure out how to import plugins into the project. Now all user has to do is to append `pelicanext.latex.latex` to his PLUGINS array.