hook for save events so that gitbacked plugin registers the new .png files

[fama]

Hello,

I installed this plugin and it's great! One thing I noticed is that the gitbacked Plugin does not detect the new .png files created by the extension and thus they do not get backed up. Would it be possible to fire some event to notify gitbacked plugin to register the save event.

woolfg suggested that it may be the cleanest way to register the new .png files and .png file changes (issue 51)

I would love to do a pull request, but the coding is beyond my skills.