jsonhttp: config enable CORS

[t-bast]

CORS requires options calls to be handled on all routes, so I think that the easiest way is to automatically add an OPTIONS handler on all defined routes when CORS is enabled.

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This change is 

[codecov]

Codecov Report

Merging #476 into master will decrease coverage by 0.19%.
The diff coverage is 27.27%.

@@            Coverage Diff            @@
##           master     #476     +/-   ##
=========================================
- Coverage   70.32%   70.12%   -0.2%     
=========================================
  Files          90       90             
  Lines        5327     5346     +19     
=========================================
+ Hits         3746     3749      +3     
- Misses       1208     1219     +11     
- Partials      373      378      +5

Impacted Files	Coverage Δ
fossilizer/fossilizerhttp/cmd.go	0% <0%> (ø)	⬆️
store/storehttp/cmd.go	0% <0%> (ø)	⬆️
jsonhttp/jsonhttp.go	70.32% <33.33%> (-9.94%)	⬇️

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[alexppxela]

Reviewed 4 of 4 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @bejito and @stephan83)