new(xychart): add colorAccessor to relevant series

[williaster]

🚀 Enhancements

📝 Documentation

This closes #996 and adds the ability to override color via colorAccessor to the following series that should feasibly support per-datum colors (you can already override colors for Line/AreaSeries)

• BarSeries
• GlyphSeries
• BarStack
• BarGroup

It also

• adds tests for new functionality
• updates the /xychart demo to show this functionality (annotated datum gets a pattern color)
• updates AnimatedGlyph/Bars to handle url(#...) based colors (hadn't tested this before and react-spring chokes on them; have a solution that looks okay tho)

@hshoff

[coveralls]

Pull Request Test Coverage Report for Build 472664628

• 10 of 11 (90.91%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 61.33%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/visx-xychart/src/components/series/private/AnimatedGlyphs.tsx	1	2	50.0%

Totals
Change from base Build 466988844:	0.1%
Covered Lines:	1735
Relevant Lines:	2697

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💛 - Coveralls