Switch Assemble over to their current Paradigm

While @ajturner has been working on #215 we've come across another issue that I predict will become increasingly tedious. Essentially since our templating language is handlebars, and Vega uses modified handlebars templates, things like: `"template": "{{parent.value|number:'.2f'}} ºC"` will cause Assemble to error with its current version. Meaning we have to sub-optimally alter specifications to work around Assemble.

Right now we are using Assemble v0.4.37 with Grunt. Apparently this is around when Assemble stopped supporting Grunt, so if we want to update Assemble we need to switch over to how Assemble currently does its things. Which is its own custom Assemblefile. Yay.

Thoughts @tomwayson?