Add --when option

[k1LoW]

No description provided.

[codecov]

Codecov Report

Merging #248 into master will decrease coverage by 0.69%.
The diff coverage is 80.35%.

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
- Coverage   68.26%   67.57%   -0.70%     
==========================================
  Files          20       22       +2     
  Lines        3577     3633      +56     
==========================================
+ Hits         2442     2455      +13     
- Misses        894      938      +44     
+ Partials      241      240       -1     

Impacted Files	Coverage Δ
cmdutil/when.go	65.62% <65.62%> (ø)
cmdutil/option.go	100.00% <100.00%> (ø)
datasource/datasource.go	22.22% <0.00%> (-12.70%)	⬇️

---

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 ae5b79c...1e78e88. Read the comment docs.