Detect and collate redundant shape usages in the sprite

[jkphl]

There are situations where one particular SVG shape needs to be present multiple times in the sprite, e.g. when you have a particular icon and several different hover states for it. Right now, you need to have several copies of the shape, each with its individual hover state. It would be nice if iconizr could detect these redundant icons (e.g. by file hashes) and put <use> elements for the shape copies in the sprite instead (would save some size).