Add `grunt precommit` for running front-end dev and test tasks before commit.

This includes autoprefixer, imagemin, jshint, and qunit.

props jorbin.
fixes #27121. see #25169 and #27078.


git-svn-id: https://develop.svn.wordpress.org/trunk@27176 602fd350-edb4-49c9-b593-d223f7449a82