Disable animated gif generation

I've added a command line parameter which allows you to disable animated gif generation. The default behaviour is the same as before, but adding the `--no-animations` switch will stop animated gifs being created.

This is useful to us because the Mac Mini we use as a CI server takes a long time to generate the animations each time, and to us they are not entirely necessary.

If there's any style problems then let me know and I'll fix them :+1: