Core features complete

New features:

• Support for multi-line nodes.
• Edge styles, multi-segment descents for level skipping.
• Wrote a manual.
• Tree annotations: movement arrows, constituent highlighting / underlining.
• Complex figures: grids of trees, captioning.

Fixes, improvements, changes:

• Massively improve responsivity of svgling diagrams.
• TreeLayout objects manage font size directly.
• distance_to_daughter is now a distance between levels, i.e. excluding node
  height.