Add LoggerWithFormatter method

[sairoutine]

Problems

I want to define gin's log format.
Currently, instead of using the default Logger, I can define the log format by defining my own Logger function.

However, my own Logger function must include redundant codes similar to the default Logger.

ref. #1581

Proposal

This PR solves the problem by adding new methods, LoggerWithFormatter and LoggerWithConfig.

example code

r := gin.New()
r.Use(Recovery())
r.Use(LoggerWithFormatter(func(param gin.LogFormatterParams) string {
	// my own format
	return fmt.Sprintf("[GIN] %v | %3d | %13v | %15s | %-7s  %s | %s\n%s",
		param.TimeStamp.Format("2006/01/02 - 15:04:05"),
		param.StatusCode,
		param.Latency,
		param.ClientIP,
		param.Method,
		param.Path,
		SomeMyOwnParam(param.Request), // username or payload information
		param.ErrorMessage,
	)
}))

refs.
gorilla's LogFormatter
http://www.gorillatoolkit.org/pkg/handlers#LogFormatter

echo's LoggerWithConfig
https://echo.labstack.com/middleware/logger

At end

If you prefers to this proposal, I will add unit tests and documents.

Thanks.

[codecov]

Codecov Report

Merging #1677 into master will increase coverage by 0.2%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1677     +/-   ##
========================================
+ Coverage   99.19%   99.4%   +0.2%     
========================================
  Files          41      41             
  Lines        1995    2017     +22     
========================================
+ Hits         1979    2005     +26     
+ Misses         12       9      -3     
+ Partials        4       3      -1

Impacted Files	Coverage Δ
logger.go	100% <100%> (+5.06%)	⬆️

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[thinkerou]

@sairoutine Hi, can you see the log middleware https://github.com/gin-contrib/logger ? thanks!

[sairoutine]

@thinkerou
Yes, I've seen it but it is specialiced in rs/zerolog. this pull request provides a more general logging that the web application framework should have.

Also, by this pull request, Your suggested middleware will be the simpler code and it's redundant code will be eliminated.

Thanks.

[thinkerou]

@sairoutine please commit the whole code which includes unit test and example code, thanks, and then we review.

[sairoutine]

@thinkerou
Added unit test and README 👍
Please review. Thanks.

[sairoutine]

Thank you for review and merge!