Svgling

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

New feature:
- Compatibility mode aimed at Inkscape: allow using `px` instead of relative
`em` units. This allows current versions of Inkscape to load the output
SVGs. This mode is not on by default.

Fixes, improvements, changes:
- Update documentation to reflect some issues in MathJax math mode delimeter
choice for hybrid mode.

New features:
- Basic support for drawing trees using a mix of HTML/CSS (for positioning) and
SVG (for line drawing). This is much more limited than the core drawing
algorithms, but allows for arbitrary node labels + MathJax.

Fixes, improvements, changes:
- Improve documentation

New features:
- Support for multi-line nodes.
- Edge styles, muti-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.

- Fix for non-leaf nodes that are larger than the leaf nodes they dominate.
- More flexible arguments to `draw_tree` (will accept the top node + children
via variable arguments, rather than a list/tuple as the first argument.)
- Documentation improvements.

- Support for basic tree drawing with lists and `nltk.tree.Tree` objects.