feat(ng-include): remove inline html support

BREAKING CHANGE: This was not really in usage in the wild and not
supported by AngularJS.  `ng-include` attribute values must now always
resolve to URLs.

Closes #127
docs(ng-include): add documentation

Closes #127
fix(ng-include): require {{ }} for interpolation

BREAKING CHANGE: `ng-include` attribute values are no longer
interpolated by default.  To use interpolation, you *must* surround them
in `{{` / `}}` pairs.

| Before                               | After                                    |
|--------------------------------------|------------------------------------------|
| `<div ng-include="'template.html'">` | `<div ng-include="template.html">`       |
| `<div ng-include="obj.templateUrl">` | `<div ng-include="{{obj.templateUrl}}">` |

Closes #127