exportHTML.js cleanups

[rhansen]

Multiple commits:

• Avoid awkward comments.comments
• Load HTML into a throwaway div instead of calling cheerio.load()
• Avoid redundant context.lineContent assignments
• Avoid evaluating $(this) multiple times
• getLineHTMLForExport: Use cheerio for all HTML manipulations
• exportHTMLAdditionalContent: Use cheerio to build HTML
• Move function definition above its use

[JohnMcLear]

I like this approach better. I was lazy before.