chartlet 0.7.0

Wider plots for time charts and a unit at the top tick. The specification keeps
schemaVersion: 1; every specification that rendered with 0.6.0 still renders, with the
changes in output listed under Changed.

Added

• Time charts and small multiples: valueAxis.unit, the unit after the top tick label and the
  data table's column names, instead of a title above the plot.

Changed

• Every time chart makes its value-axis gutter as wide as its widest tick label needs, at most
  the former 72 pixels, so the plot is wider; time tick labels at the left edge move inward like
  those at the right edge.