Yaml binding

[doxxx]

This adds binding support for YAML.

Sidenote: testBodyBinding uses JSON.Bind instead of the Binding instance passed as a parameter when testing the badBody. Is this intended?

[codecov]

Codecov Report

Merging #1618 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1618      +/-   ##
==========================================
+ Coverage   99.27%   99.28%   +<.01%     
==========================================
  Files          39       40       +1     
  Lines        1931     1948      +17     
==========================================
+ Hits         1917     1934      +17     
  Misses         10       10              
  Partials        4        4

Impacted Files	Coverage Δ
binding/yaml.go	100% <100%> (ø)
binding/binding.go	100% <100%> (ø)	⬆️
context.go	99.49% <100%> (ø)	⬆️

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[doxxx]

I'll take a look at improving the test coverage in the next day or so.

[thinkerou]

@appleboy please update review comment and then LGTM, thanks!

[appleboy]

@thinkerou Sorry. The external package has already in go.mod.

[thinkerou]

@appleboy yes, no sweat.