Add mutation of checkbox element on ref and value change

[andyrichardson]

Fix #28

[codecov-io]

Codecov Report

Merging #29 into master will increase coverage by 0.08%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   91.71%   91.79%   +0.08%     
==========================================
  Files           3        3              
  Lines         181      195      +14     
  Branches       63       66       +3     
==========================================
+ Hits          166      179      +13     
- Misses         12       13       +1     
  Partials        3        3

Impacted Files	Coverage Δ
src/useField.ts	95.45% <92.85%> (-0.7%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4d78973...de9fdbb. Read the comment docs.