Node styling and compatibility

0.4.0 - Node styling and compatibility

As of 0.3.1, svgling is the default tree-drawing package for nltk Trees
in Jupyter; this update provides better compatibility for that. It also
improves some styling options, and direct support for format conversion.

New features:

• Limited per-node styling in trees: supports changing font style, size, and
  color/fill
• convenience wrappers to make conversion to PNG/PDF via the cairosvg
  package easier (cairosvg is now an optional package dependency)
• Support nltk.Tree objects directly in svgling.figure classes

Fixes, improvements, changes:

• Simplify import code for nltk, remove monkeypatching
• The option global_font_style is renamed to just font_style
• Refactor TreeOptions to make it easier to pass around options bundles
• Default change: relative_units=False. This provides better compatibility
  with inkscape, among other things. This option is now deprecated.
• Documentation improvements

(Note: this version actually released 25-09-2023, just forgot to mark the tag as a release on github at the time.)