Read more link plugin

I made this as a pull request to the original pelican project when pelican-plugins wasn't created yet.  I was told to convert and move it to the plugin repository. This is a simple plugin that I use on my own blog.  Just thought I'd share.  Here is an extended read more on this: http://vuongnguyen.com/creating-inline-read-more-link-python-pelican-lxml.html