Allow declaring expressions as 'safe'

[MaKleSoft]

I understand why Polymer is escaping source code in expressions by default, but it should also be possible to declare an expression as 'safe' and allow it to return source code, as in other frameworks.

I like the handlebars triple mustache syntax.

<div>{{{ safeExpression }}}</div>

[jmesserly]

thanks for the help on the other bug @MaKleSoft ... your analysis of the problem is right on. I'm going to merge this in with a similar bug to keep them together: googlearchive/TemplateBinding#57