middleware: add assets handler

[veezhang]

• add middleware pkg with assets handler
• 100% test coverage

[codecov-commenter]

Codecov Report

Merging #17 (2a7fae0) into master (97328dd) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #17   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        14    +1     
  Lines          624       689   +65     
=========================================
+ Hits           624       689   +65     

Impacted Files	Coverage Δ
middleware/assets.go	100.00% <100.00%> (ø)

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[nianiaJR]

LGTM