Added automatic figure numbering to Better Figures & Images Plugin

This adds Automatic Figure Numbering to the Better Figures & Images plugin - see [here](http://duncanlock.net/blog/2013/10/19/the-better-figures-images-pelican-plugin-now-supports-figure-numbering/) for more info.

I rebased this as per [this](https://github.com/getpelican/pelican/wiki/Git-Tips#squashing-commits) but it seems to have rebased this onto a commit from Justin? I don't know - I hate git rebase.

Anyway, the diff and commit message both seem to be correct, but since I rebased, github says it's can't auto-merge and I got conflicts (only with my own changes) during the rebase (which I resolved). Sigh. If this won't merge cleanly and you need me to do stuff, just let me know.