Add ability to set the logger on echo.Context

[Cyberax]

This change allows middleware to replace the logger on the echo.Context
with a customized per-request logger with additional fields. The logger
is reset to default on every Reset() call.

[codecov]

Codecov Report

Merging #1377 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1377      +/-   ##
=========================================
+ Coverage   84.06%   84.1%   +0.03%     
=========================================
  Files          27      27              
  Lines        2065    2076      +11     
=========================================
+ Hits         1736    1746      +10     
  Misses        215     215              
- Partials      114     115       +1

Impacted Files	Coverage Δ
context.go	90.94% <100%> (+0.22%)	⬆️
echo.go	85.62% <0%> (-0.1%)	⬇️

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