Add new thumbs config
Merge pull request #2370 from Pinpickle/new-thumbs-config

Add new thumbs config
Update changelog.md
Sanitize width/height values to int
Calculate w/h values if one of them or both are set to '0' preserving aspect ratio
Little string changes
Concat the PHP way
Direct return
Switch logic
Add documentation
Changelog
Merge pull request #2375 from rarila/issue-2372

Automatic proportional scaling for twigs showImage()