Draw droplines to the x and y axes on hover if the option is enabled and we're not on 'compare' hovermode.

Added axis layout option for showspikes
Added background behind droplines
Added axis indicator marker, including on free anchored axes
Always draw dropline to chart limit, including for shared axes - not drawing all the way to free anchored axes

Move 'dash' function to lib for re-use
Use dynamic contrast color based on same logic as hovertext contrast text

Make single attr calls
Add crisp style
Fix edge case when leaving and returning to closest hovermode

Keep spike background color to the chart background, unless the series color is too close, then use contrast color

Fix modebar tests, and add some new ones
Add spikeline tests
Final adjustment to marker placement to account for y-axis on right side

Fix 'across' for right-side axes.

- toaxis always goes from the point to the axis, even for free axes
- across spans all counteraxes with subplots for this axis, and
  extends to the free axis if it exists
- fixed a bug with positioning after scrolling gd within a container
- also short-circuited calculations we don't need

…t the pattern

I had to make a mock marker.line object in _fullData to keep legends happy,
but at least it doesn't respond to values in data.