feat($interpolate): enable escaping interpolated expressions

Previously, Angular would offer no proper mechanism to reveal attempted script
injection attacks when users would add expressions which may be compiled by
angular.

This CL enables web servers to escape escaped expressions by replacing
interpolation start and end markers with escpaed values (which by default are
`{{{{` and `}}}}`, respectively).

This also allows the application to render the content of the expression
without rendering just the result of the expression.

Closes #5601