WIP - fix(*): make templates work with custom interpolation symbols

Previously, components specifying their own templates (such as dialog,
toast) assumed that the interpolation start-/endSymbols where `{{`/`}}`,
thus breaking in apps using custom symbols.
This commit introduces a new `$mdUtil` method for replacing the "assumed"
symbols (`{{`/`}}`) with the actual ones.

Closes #877

**NOTE:**
This is still work-in-progress. It only fixes Toast. Still pending:

* Dialog (alert, confirm)
* TextField (seems to work as is, but...)