feat($interpolate): use custom toString member if present

BREAKING CHANGE: use custom toString implementation when present on object value types.
Behavior is consistent with implementations found in other languages such as Ruby, Python, and CoffeeScript.
http://rubymonk.com/learning/books/1-ruby-primer/chapters/5-strings/lessons/31-string-basics
https://docs.python.org/2/library/stdtypes.html#string-formatting-operations
http://coffeescriptcookbook.com/chapters/strings/interpolation

Closes #7317