Commit node_modules

:bomb: 

Why?
----

 * Considered best practice - http://www.futurealoof.com/posts/nodemodules-in-git.html
 * Stops `imagemin` (though not exclusive to this module) failing our build when it can't be found
 * Speeds up build (infinitesimally)

Why not?
----

 * Increases repo size


Thoughts?