Add stub for thumbnail plugin, just mimics image plugin for now
Thumbnail directive produce expected image and thumbnail URIs
Thumbnail directive can imitate Figure as well as Image directives
Move methods from Galleries plugin to new ImageProcessor class

Makes resize_image() and image_date() methods reusable in other plugins.
Add scale_images task plugin
Move default list of known image extensions from Gallery to ImageProcessor
scale_images plugin only processes files with known image extension
Mention IMAGE_FOLDERS option in default conf.py template
Add copyright notice to thumbnail and scale_images plugins
Remove unneeded imports and variables from image_processing.py
Fix style consistency issues to pass flake8
Add info about scale_images and thumbnail plugins to docs and changes
Remove import no longer used in galleries plugin
Merge pull request #1527 from pellenilsson/thumbnail-directive-rebased

Thumbnail feature