Restructured grunt requirejs task options to have defaults for most of them and only specify options
where we need to override.

Then created onefile task which can be used to build a single-file build for any single module with
all of its dependencies (e.g. "grunt onefile:WinJS/Promise" or "grunt onefile:WinJS/Controls/AppBar").

Used onefile task to build base.js and the single-file WinJS.js because those are canonical instances.