Image Picka per eight04

There is a bug preventing extensions from downloading images from cache. More information: https://github.com/eight04/image-picka/issues/270

An image picker written in webextensions.

Features:

* Download an image with drag-n-drop or ctrl/shift/alt + click.
* Show download button on hover.
* Batch download all images from current tab or tabs to the right.
　　- Filter images according to
　　　　- min width
　　　　- min height
　　　　- file size
　　　　- image URL
　　- Copy URLs of selected images.
* Customized filename.
* Handle invalid characters in filename.
* Download from browser cache.
* Transform URLs with find-and-replace.

For more information, visit https://github.com/eight04/image-picka.