Add support for one way filters

Filters are written as:

<template bind>
  {{ user.age | filter1 | filter2() | filter3(21, 'Hi') }}
</template>

To register a filter do:

PolymerExpressions.filters.filter3 = function(n, s) {
  return {
    toDOM: function(value) {
      if (value >= n)
        return s + '! Have a beer.';
      return 'Come back in ' + (n - value) + ' years';
    }
  };
};

BUG=
R=rafaelw@chromium.org

Review URL: https://codereview.appspot.com/12926044