Added MAKER styles by default, including nearly-black background.  Based on MAKER tiers styling from Apollo.  Also added custom_track_styles.css, stub for customization.
Added hook for per-feature filtering when rendering tracks.
Defined standard feature filters for filtering by strand.  Need to generalize soon so tracks can be passed other filters.
Added option to toggle between filtering to display just plus strand annotations, just minus strand annotations, or both.  Needs further work on filter implementation in HTMLFeatures, because tiers aren't resizing to shrink when plus/minus strand annotations filtered out.
Adding arrowheads designed for new track background colors.  Note that not doing generic transparency for arrowhead png background because anti-aliasing intermediary opacities might slow down rendering.