Add ValidateBinder as an optional Binder implementation

[Tethik]

My attempt at #1105

I added a new Binder implementation ValidateBinder that basically calls context.Validate after successful bind. You enable it like this:

e := echo.New()
e.Binder = &echo.ValidateBinder{}
e.Validator = ...

Then later in your handler when you call context.Bind the ValidateBinder will call both bind and validate.

I added some tests based on the existing ones for bind. Also used this as a test implementation:
https://gist.github.com/Tethik/ef55db82e82fc662262245949593deea

Somewhat newbie at Golang, feedback and suggestions would be greatly appreciated :)

[codecov]

Codecov Report

Merging #1144 into master will increase coverage by 0.53%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1144      +/-   ##
==========================================
+ Coverage    81.3%   81.83%   +0.53%     
==========================================
  Files          25       25              
  Lines        1904     1905       +1     
==========================================
+ Hits         1548     1559      +11     
+ Misses        249      242       -7     
+ Partials      107      104       -3

Impacted Files	Coverage Δ
bind.go	85.45% <100%> (+0.36%)	⬆️
middleware/logger.go	84.46% <0%> (-0.44%)	⬇️
middleware/middleware.go	100% <0%> (ø)	⬆️
echo.go	87.9% <0%> (+0.26%)	⬆️
context.go	75.45% <0%> (+1.81%)	⬆️
middleware/jwt.go	88.57% <0%> (+6.99%)	⬆️

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