Ziaplot

- Added `NumberLine` graph type
- Added a zorder parameter for layering drawing elements
- Allow named font sizes, such as "large" and "small", in CSS
- Change arrowhead marker to point at its data coordinate rather than be centered over it

This release includes major BREAKING CHANGES. The API was reworked to focus more on
graphing of geometric primitives (Lines, Circles, Points, etc.) in addition to empirical data.
It also adds a CSS styling system.

- Added geometric objects and shapes, including `Line`, `Point`, `Circle`, `BezierQuad`, `Function`, etc., in the `geo` submodule.
- The old `Line` has become `PolyLine`, with alias `Plot`.
- The new `Line` represents a true Euclidean line rather than a set of (x, y) coordinates connected by line segments.
- Added `LayoutGrid` for placing axes in a grid. `LayoutH` and `LayoutV` now inherit from `LayoutGrid`, and no longer support nested layouts.
- Added `HistogramHoriz` for horizontal histograms
- Added `Implicit` to plot implicit functions
- Renamed axes to `Graph`, `GraphQuad`, `GraphPolar`, `GraphSmith`. `Diagram` added as an empty surface for geometric diagrams.
- Added tangent and normal lines.
- Added point placemenet at intersections and extrema.
- Added `ticker` for easy forming of tick locations
- Added `equal_aspect` method to equalize x and y scales
- Changed to use a CSS-like styling system for applying styles and themes.
- Added more legend location options

- Updated Pie and Bar interfaces
- Fixed legend text spacing
- Added context manager for axes and layouts
- Allow more than 2 colors in colorfade
- Added contour plots
- Added LayoutGap for leaving space in a Layout

- Implement ziamath's Text object for plot labels
- Added size property to plot style, combining canvas width and canvas height
- Added global configuration object

- Added an option to use SVG1.x format for compatibility since SVG2.0 is not fully supported in all browsers/renderers yet.

- Use ziamath library to draw math text.