Dev tools: Add `grunt imagemin:core` task for optimizing images pre-commit.

Adds grunt-contrib-imagemin, so an `npm install` will be required.

props jorbin, wonderboymusic for initial patches.
see #25169.


git-svn-id: https://develop.svn.wordpress.org/trunk@27172 602fd350-edb4-49c9-b593-d223f7449a82
Optimize 69 images (of 127) using the new Grunt task. (See [27172].)

fixes #25169.


git-svn-id: https://develop.svn.wordpress.org/trunk@27173 602fd350-edb4-49c9-b593-d223f7449a82