assets: require.js and uglifyjs support

* Enables `requirejs` and `uglifyjs` filters on webassets.
* Updates Flask-Assets/webassets to 0.10(.dev)

We are now able to embrace structured ([AMD](http://requirejs.org/docs/whyamd.html)) JavaScript modules and applications.

* see #1794
* discussion (or the lack of) https://forum.invenio-software.org/t/webassets-vs-grunt/45