Implemented ShouldRenderFile

Please review.
FYI These changes were a good candidate for a separate PR since it's a separate logical change and could be merged in a lot quicker since it's such a simple change. No big deal though - it's already in here, just good food for thought :) by the way - if you get a spare moment at some point feel free to check out my presentation about pull requests: https://github.com/robdmoore/PushBetterSoftwareWithPullRequestsPresentation

It's focussed on commercial development, but there are still some good general tips in there :)