Comments and headings fixes

• Empty top level directives no longer break the header indexer
• Block comments without a space after the .. are no longer recognized.
• Empty comments don't cause indented text to be highlighted as comments

Fixes #24
Fixes #25
Fixes #31
Fixes #32

Thanks to @lextm, @ssbarnea and @Xeverous for contributions to issue tracking.
Thanks to @alcarney for contributing a test suite.
Thanks to @sgryjp for fixing the empty top level directories.