Custom plugins

Hi there,

This is a duplicate of the Pull Requests on the main pelican-plugins repo:

* [Update readme. Include BeautifulSoup requirement and usage](https://github.com/getpelican/pelican-plugins/pull/212)
* [Added Image metadata attribute for article to specify a custom image ](https://github.com/getpelican/pelican-plugins/pull/218)

Not sure who monitors the Pull Requests over there. Are you able to review them or this PR and then push back to the main plugins repo?

Happy for feedback. I was about to write my own version and was happy to see someone had beaten me to it!

Cheers,
Jesse