Allow custom formatting of date/time columns in CSV export

[bladams]

Fixes #98

[codecov]

Codecov Report

Merging #106 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   87.48%   87.53%   +0.04%     
==========================================
  Files          17       17              
  Lines        4804     4821      +17     
  Branches      369      370       +1     
==========================================
+ Hits         4203     4220      +17     
  Misses        544      544              
  Partials       57       57              

Impacted Files	Coverage Δ
webgrid/tests/test_rendering.py	99.85% <100.00%> (+<0.01%)	⬆️

---

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 bf13a3e...514cef6. Read the comment docs.