Allow script tags to have custom attributes
This is primarily intended to be used with async and defer attributes which allow non-blocking rendering in the browser and defered execution.
It could also allow, once implemented, to have custom attributes on script tags which are popular in the form of data attributes (popularily is the requirejs data-main tag).
I felt like asset handling in-core wise needed a little love and this is intended to be a first step.
I'll upload a WIP changeset to Gerrit soon.