Fix issue described at http://stackoverflow.com/questions/21777058/loop-index-in-included-nunjucks-file

http://stackoverflow.com/questions/21777058/loop-index-in-included-nunjucks-file

`scanLoop` was not finding the usages of `loop.index` etc. because they were inside included files.

I'm not sure this is the best way to solve this, but scanning every included file (and files they include themselves) seems overkill. Any thoughts?