chartlet 0.4.0

Compact charts, panel notes and step ends. The specification keeps schemaVersion: 1; every
specification that rendered with 0.3.0 still renders, with the changes in output listed under
Changed.

Added

• Compact charts from 200 pixels wide (mobile variants too): below 320 pixels the value-axis
  gutter and the margin are narrower, so a panel rendered at its width keeps its text size.
• Small multiples: panes[].note, a short finding under a panel title.
• Step lines: stepEnd, where the last step ends without an observation of its own.

Changed

• Small-multiples panel titles take two lines before they are shortened.
• A declared timeAxis.precision changes how times are named, no longer the tick spacing.