Support angular/django style filters

[bgourlie]

For my use-case specifically, filters would ideally be able to insert nodes into the DOM, for example:

<div>{{ myContent | html }}</div>

Where html is a filter that converts a string value into a node or document fragment.

[bgourlie]

I submitted a bug over at dart's bug tracker for the same issue. They have reworded the title and it may make more sense: https://code.google.com/p/dart/issues/detail?id=14234

[sorvell]

We cannot support this directly for security reasons; however, <template if> or a custom element can be used to output dom.