Since developers are allowed to customize start/end interpolation
strings, but third-party directive creators don't know about these
customizations, we should standardize on {{ }} in templates of
reusable (third-party) directives. During the compilation, these
templates are then denormalized to use whatever the custom
start/end symbol is, effectively translating the template into the
syntax of the runtime environment.
This addresses an issue raised at http://goo.gl/e8VPV
Existing code should not be affected by this change since project
that do use custom interpolation markers are not expected to use
{{ }} in existing directive templates.