Added option for sub-sections

Content Changes: Each section is now wrapped in a <section> and all sub-sections wrapped in a <div>. This makes sorting easier and should fix some of the highlighting issues

Nav Changes: The sub-sections are setup in a child <ol> to the the parent section's <li>
Merge branch 'refs/heads/feature/add-sub-sections' into feature/fix-scrolling

* refs/heads/feature/add-sub-sections:
  Added option for sub-sections
  Added option for sub-sections
Fixed active state when scrolling

Plugin now checks to see if each <section> is within visual range as set by "scrollOffset" in settings. If multiple sections are within view, all are set as "active"
Merged changes from PR #13
Added a wrapper to fix #17

Also moved the function appending the headline back to the nav setup
Fixed console message
Merge pull request #16 from jimmynotjim/feature/fix-scrolling

Fixed active state when scrolling