vellumplot 0.3.0

• Adopted vellum's renamed vl_* graphics primitives (vl_gpar(), vl_unit(),
  vl_viewport()), which no longer mask grid.

• NA legend keys for size / shape. A mapped size or shape aesthetic
  whose data contains missing values now shows an "NA" key (as the colour scales
  already did). Also fixes a crash: NA in a shape mapping previously errored
  (Unknown point shape: NA) — it now draws as a neutral circle.

• 2-D density contours. mark_contour() draws iso-density contour lines of a
  2-D point cloud and mark_contour_filled() fills the bands between them
  (coloured by level automatically). By default the field is a kernel density
  estimate (via MASS::kde2d()); map a z aesthetic to contour a supplied
  surface over a regular x/y grid instead. Contour tracing uses the isoband
  package (both isoband and MASS are Suggests).

• Binned position scales. scale_x_binned() / scale_y_binned() cut a
  continuous axis into bins — ticks at the bin boundaries, each datum drawn at its
  bin centre — reusing the binned-colour classification (style/n, or explicit
  breaks). mark_bar() sizes to the bin width.

• Rich and multi-line text labels. mark_text() now carries rich labels
  through to the renderer instead of flattening them: map label = md(<expr>) for
  a per-datum styled label (bold/italic/super-/subscript/colour), and plain labels
  may contain newlines (\n) to wrap onto stacked lines. Requires vellum's
  development version. (mark_label()'s background box does not yet support rich
  labels.)

• Accessibility (alt text). Every compiled plot is now an accessible image
  by default. plot_alt() returns a plot's text alternative — an author-written
  string from the new labs(alt = ), or a prose summary vellumplot generates from the
  spec (chart type, x/y/colour/size mappings, observation count, faceting). At the
  compile seam the plot title becomes the scene's accessible name and this alt
  text becomes its description, so render_plot() output carries role="img" +
  <title>/<desc> in SVG and a tagged Figure with Alt in PDF (via vellum).
  See the new Accessibility article. Requires vellum's development version
  (>= 0.1.1.9000) for the accessible SVG/PDF backend.

• British spelling. mark_*(colour = ) is now honoured as an alias for
  color (previously the mapping was silently dropped and no colour scale was
  trained). Added scale_colour_*() aliases for the scale_color_*() family.

• Faithful alt text. The auto-generated description now matches what the plot
  actually renders: it honours scale_*(name = ) axis/legend titles, names the
  implicit "count" axis of a count bar, and describes a network graph by its node
  and edge counts instead of its (meaningless) layout x/y axes.