Adding Javascript Task to the demo

[zekth]

Just adding Javascript task with concat and sourcemaps which kind of a boilerplate like it was done with pug templates and less stylesheets. Don't think it overload the front page.

[phated]

@zekth awesome!! I like that you added sourcemaps because it'll look great when we convert the example to gulp 4. The small change I'm going to make is sourcemaps.write('.') to sourcemaps.write() (for inline instead of external maps).