Merge of new validator with tests

Per @gvwilson request (issue #34).
Make files are back on the plan? For what it's worth, I was able to run it from the pages directory using:
*../tools/validate_markdown_template.py single index.md home*

And from the lesson-template directory using:
*tools/validate_markdown_template.py single pages/index.md home*

I've pushed a commit with a much-simplified default assumption: when validating links, it looks for markdown in the same folder as the input file being validated. The old convoluted expression is still there, but just to support unit tests that aren't reading from a file.