Loop Helpers

Now we can use loop helper object in each like Django template.

Example:
{{#foo}
  {{loop.counter}} {{! The current iteration of the loop (1-indexed) }}
  {{loop.counter0}} {{! The current iteration of the loop (0-indexed) }}
  {{loop.revcounter}} {{! The number of iterations from the end of the loop (1-indexed) }}
  {{loop.revcounter0}} {{! The number of iterations from the end of the loop (0-indexed) }}
  {{loop.first}} {{! First object of the foo }}
  {{loop.last}} {{! Last object of the foo }}
{{/foo}