Grunt: move tasks/version.js to a grunt plugin

[retlehs]

let's move tasks/version.js out of the repo and publish it as a more generic version updater for wordpress enqueued CSS and JS

you should be able to define a file to be updated (lib/scripts.php for us), and any CSS + JS assets that should be versioned by providing the handles (roots_main, roots_scripts)

this should be a repo on the @roots org and published to npm. maybe grunt-wp-asset-ver?

/cc @sjothen