chartlet 0.5.0

Small panels and their ticks. The specification keeps schemaVersion: 1; every specification
that rendered with 0.4.0 still renders, with the changes in output listed under Fixed.

Added

• Time charts: timeAxis.step, the distance between time ticks — whole years on a calendar
  axis, units on a numeric one.
• Small multiples: noteEmphasis sets a panel's note apart, in the text color and semibold.
• Charts from 160 pixels tall (mobile variants too), for small panels.
• PNG of the mobile variant: --variant mobile --format png, with the literal colors of the
  print variant.

Fixed

• A time tick label that would run into the one before it is left out; its gridline stays.