[xy-chart] add support for confidence bands

[williaster]

This PR

• adds support for confidence bands in the <AreaSeries /> component and updates the readme + tests. To implement this it bumps to @vx/shape@0.145 which added support for <Area /> components
• adds support for labels in <HorizontalReferenceLine />
• bumps to @vx/axis@0.145 which has adds less-strict tickValue props
• preserves user-set onMouseMove and onMouseLeave props for all child series instead of having XYChart overriding them. this also makes it possible to disable mouse events for specific subsets of child series by setting these handlers to null
• refactors imports of @vx components to absolute imports to help with bundle size (addresses use absolute imports for vx components, update builds to support absolute imports for data-ui components #49 for this package)

[coveralls]

Coverage decreased (-0.06%) to 84.856% when pulling 98de4a4 on chris--band-series into 74e88a2 on master.

[coveralls]

Coverage decreased (-0.2%) to 84.697% when pulling 98de4a4 on chris--band-series into 74e88a2 on master.

[coveralls]

Coverage decreased (-0.2%) to 84.703% when pulling 3f3e2b1 on chris--band-series into 74e88a2 on master.