error checking and handling

[gregcaporaso]

• if there is markdown in a heading (i.e., md line starting with #) the heading will be cut off at the start of the markdown. if heading starts with markdown, the build will fail with a non-informative error message
• if there is more than on top-level heading (i.e., single #) in a single document, all content beginning with the second top-level heading will dropped from the output
• if a header is missing a link tag the build fails with the following traceback: AttributeError: 'IDFinder' object has no attribute '_ID'.