Restructure syntax highlighting

[evanp]

Our syntax highlighting is kind of a mess. I'd like to get it so we have the following elements available for styling:

• x ... done task appears as a comment
• creation date
• completion date
• (A) priority
• +Project
• @Context
• description plain text
• due:yyyy-mm-dd due date
• t:yyyy-mm-dd start date
• key:value key-value metadata pairs

I can see adding other special handling for well-known key-value pairs, but for now this should probably work well.