Fix errors related to encoding of missing music

[dchiller]

Adds fixes to the script for two cases:

1. Two few hyphens between 6's in a volpiano string would cause the missing music section not to be recognized.
2. Line, page, or column breaks after a missing music section would lead an error to be thrown on attempted alignment.

Fixing these cases provided an opportunity to simplify how the script deals with spacing around missing music and barlines:

• "proper" spacing is no longer enforced during preprocessing or during sectioning
• all spacing is now done in the postprocessing step

In the course of making these fixes, also discovered a way to remove a branch in the _align_section function

Closes #19

Also, provides an update for the test case visualization html that makes it easier to view long test cases.

[dchiller]

I see that at least some of your functions are labelled by what they return, but all of the variables in this pull request are unannotated.

Goooood catch....this is why we do code review!!! :)