Add image optimization pre-commit plugin

Hi Causes team,

Thanks for the awesome gem! I wrote a plugin in order to optimize images during pre-commit. You might find it useful.

Thanks,
George

Edit: adding checklist:

* [x] 1.8.7 support
* [x] Remove the image_optim bundler dependency
* [x] Exclude the plugin by default in overcommit.yml
* [x] Add jpeg to filetypes