Tailor demo page to user-selected components

This'll have the demo page that comes with the generator only include the components selected by the user.
It escapes the underscore template delimiters. The gruntfile is now [processed as a template](https://github.com/contolini/generator-cf/blob/custom-demo/app/index.js#L112) instead of simply being copied. Buuut we don't want Yeoman to process the `<%= loc.src %>` stuff, we want it to be ignored because it's actually used later by Grunt.