Feature request: optional gifsicle dependency

[mistercoffee66]

Requesting that gifsicle be a separate install for those who want to use it. This may be an edge case, but some npm installs will fail due to gifsicle without additional steps.

Please see imagemin/imagemin-gifsicle#37 (comment)

[phiresky]

As a workaround, doing this:

	new ImageminPlugin({
		...
		gifsicle: null
	}),

with package.json:

	"resolutions": {
		"gifsicle": "https://registry.yarnpkg.com/@favware/skip-dependency/-/skip-dependency-1.0.2.tgz"
	}

works

(or alternatively create a dir with an empty index.js file and do resolutions: gifsicle: "file:./path/to/empty")

imagemin/gifsicle-bin#116 would be my preferred solution though