Add support for sticky items

[clauderic]

This PR adds a stickyIndices prop to add support for sticky headers:

The prop accepts an array of indices that should be made sticky (eg, stickyIndices={[0, 20, 50, 100]})

The solution relies on position: sticky, so it will only work in browsers that support that feature, but will degrade gracefully for other browsers.

It also works with horizontal lists:

How to test locally

Clone the project and check out this branch. Run yarn followed by npm start.

To-do

• [x] Update documentation
• [x] Add test coverage for stickyIndices
• [ ] Update demo site

[codecov[bot]]

Codecov Report

Merging #55 into master will increase coverage by 0.12%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   94.16%   94.28%   +0.12%     
==========================================
  Files           3        3              
  Lines         137      140       +3     
  Branches       17       17              
==========================================
+ Hits          129      132       +3     
  Misses          8        8

Impacted Files	Coverage Δ
src/index.tsx	100% <100%> (ø)	:arrow_up:
src/constants.ts	100% <100%> (ø)	:arrow_up:

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[jeroenransijn]

Would be awesome to get this PR in, let me know if I can help at all.

[jeroenransijn]

@clauderic do you need any help getting this over the finish line? I would love to move this in the Evergreen component library.

[jeroenransijn]

👍 Thanks @clauderic!