setcolorder gains before= and after=

[dracodoc]

This was based on discussion of #4668, #4358. I first implemented a version in #4669 , then found a better implementation from ben-schwen.

I closed my PR since ben-schwen's implementation is much better. After several weeks, seeing ben-schwen may not have time to create a PR and I want users be benefited from this convenient function earlier, I'm creating this PR using his implementation with full credit go to ben-schwen.

[codecov]

Codecov Report

Merging #4691 (4a52a89) into master (457fe7b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4691   +/-   ##
=======================================
  Coverage   99.53%   99.53%           
=======================================
  Files          76       76           
  Lines       14459    14467    +8     
=======================================
+ Hits        14392    14400    +8     
  Misses         67       67           

Impacted Files	Coverage Δ
R/data.table.R	99.94% <100.00%> (+<0.01%)	⬆️

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[mattdowle]

@dracodoc Sorry for the long delay on this one. I was surprised you weren't a project member already: have invited you. What name do you prefer for the contributor list in DESCRIPTION? You don't have a name in your GitHub profile. You don't have to disclose your real name for DESCRIPTION, we can use an id there; there are two other ids there.

[dracodoc]

@mattdowle , Thanks for the invite, my name (Xianghui Dong) actually was added to contributor list long time ago :) Long time ago I made some improvements to the document when I first learned about data.table, then I had a code example for adding gzip support to fread using my work github id. Maybe my name was added from that code but I'm not sure.