Add support for nested partials; bindings on <can-import>; and source maps

[rjgotten]

Disappointed that you didn't pull features from the implementation in NetMatch/webpack-stache-loader.
As it stands, this loader is useless beyond basic usage scenarios now.

Meanwhile I'm seeing that another organization has forked that more feature-complete implementation and is ignoring this official one. Hopefully that won't have to become a trend.