Implement optional redirect when max-size is exceeded

[ewdurbin]

Description

For our use-case, we prefer to display a static image, indicating why the image is not loading, namely:

This is now one of the only divergences that aren't deployment centric in our fork of go-camo, so I'd be happy to do whatever is necessary to get it merged so we can just run from published images :)

Checklist

• [x] Code compiles correctly
• [ ] Created tests (if appropriate)
• [x] All tests passing
• [x] Extended the README / documentation (if necessary)

[dropwhile]

@ewdurbin Patch looks good. Making the new feature a non-default cli flag is great. ❤️