Allow keyboard accessible sorting

[jbinto]

We couldn't figure out a way to use the existing sortabular API to trigger sorts both by mouse click and keyboard.

This PR changes the sort function to have an onKeyDown handler by default, so you can trigger sorts using the ENTER key on the keyboard. (You can opt out of this behavior by passing disableKeyboard: true to sort.)

Let me know if you'd prefer a different approach, or if there's a better way to do this using the existing API.

[codecov-io]

Codecov Report

Merging #12 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
+ Coverage   97.94%   97.98%   +0.04%     
==========================================
  Files          10       10              
  Lines         146      149       +3     
  Branches       52       55       +3     
==========================================
+ Hits          143      146       +3     
  Misses          2        2              
  Partials        1        1

Impacted Files	Coverage Δ
src/sort.js	100% <100%> (ø)	⬆️

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[bebraw]

I don't like the flag. It would be better to inject the functionality through props. You could refactor the code as an utility and write a small example/test that does this. I agree with the idea (keyboard sorting). If you can change the design like that, I don't mind merging.