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

Previously, components specifying their own templates (such as Dialog, Toast etc) assumed that the
interpolation start-/endSymbols where `{{` and `}}`, 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
Can we use replace? If angular core assumes the default is `{{}}`, I'm sure we won't be changing it.

The main reason for this is less complexity where not needed.