Automatically optimise PNG/bitmap images

[stevejalim]

Over in https://github.com/mozilla/release-notes-media we have a workflow where an image added to the repo is automatically uploaded to an relevant cloud bucket.

We should optimise this image before upload, to ensure it's not unnecessarily large.

However, because automatic optimisation can misfire/over-optimise. We should also support the option to skip optimisation (eg via a keyword in a filename or - perhaps better - in the commit that adds the file)