cors allow regex pattern

[KlotzAndrew]

enable cors to use regex pattern for allowed origins

implementation is similar to another popular cors middleware: https://github.com/astaxie/beego/blob/master/plugins/cors/cors.go#L196-L201

[codecov]

Codecov Report

Merging #1623 into master will increase coverage by 0.12%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #1623      +/-   ##
==========================================
+ Coverage   85.08%   85.20%   +0.12%     
==========================================
  Files          28       28              
  Lines        2185     2203      +18     
==========================================
+ Hits         1859     1877      +18     
  Misses        212      212              
  Partials      114      114              

Impacted Files	Coverage Δ
middleware/cors.go	79.45% <88.88%> (+3.08%)	⬆️
middleware/util.go	90.62% <0.00%> (+6.25%)	⬆️

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[lammel]

Looking good.
A small performance penalty is expected for CORS checks due to the regex matching used.

Tests available to check for plaintext and regex matches.

[lammel]

@KlotzAndrew Thanks for your contribution!