override default CORS handler for httprouter

[Alma-media]

Related to #74

Includes support for:

• Access-Control-Allow-Origin
• Access-Control-Request-Method
• Access-Control-Request-Headers

Other changes:

• introduce ListenerOption

[codecov-commenter]

Codecov Report

Merging #102 into master will increase coverage by 0.29%.
The diff coverage is 89.10%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   83.27%   83.57%   +0.29%     
==========================================
  Files          65       68       +3     
  Lines        4276     4317      +41     
==========================================
+ Hits         3561     3608      +47     
  Misses        448      448              
+ Partials      267      261       -6     

Impacted Files	Coverage Δ
pkg/transport/http/options.go	71.64% <ø> (+1.64%)	⬆️
pkg/transport/http/listener_options.go	85.18% <85.18%> (ø)
pkg/transport/http/listener.go	75.71% <87.50%> (+1.85%)	⬆️
pkg/transport/http/middleware.go	100.00% <100.00%> (ø)
pkg/transport/http/unique.go	100.00% <100.00%> (ø)