IsTerm flag should not be affected by DisableConsoleColor method.

[sairoutine]

It is strange behavior that DisableConsoleColor turns the IsTerm flag false.

This PR contains the following changes.

• IsTerm represents whether is the output destination console.
• Default logger distinguishes color by using new method IsOutputColor.
• If ForceConsoleColor and DisableConsoleColor are called together, Later one is valid.

Note
This change breaks backward compability, but LogFormatterParams and ForceConsoleColor is not released yet. This change affects only the user which is using master branch.

[sairoutine]

What happens if ForceConsoleColor would be called after DisableConsoleColor?
This change makes the behavior easy understanding and the source code readable.

[codecov]

Codecov Report

Merging #1802 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1802      +/-   ##
==========================================
+ Coverage   98.64%   98.64%   +<.01%     
==========================================
  Files          41       41              
  Lines        2145     2146       +1     
==========================================
+ Hits         2116     2117       +1     
  Misses         18       18              
  Partials       11       11

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

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[thinkerou]

About color I think that have only two case:

1. Not use color, even isTerm=true.
2. Use color, need isTerm=true.
   so we only need one switch to indicate whether use color.
   Default use color when isTerm=true, or not use.

[sairoutine]

@thinkerou
Your suggested usecases are true. Therefore, the following usecases are seldom occured.

• If ForceConsoleColor and DisableConsoleColor are called together, Later one is valid.

This PR's purposes are two things.

• It is strange behavior that DisableConsoleColor turns the IsTerm flag false.
• disableColor and forceColor are redundant variables. To consolidate into consoleColorMode makes gin's code more readable.

[thinkerou]

@sairoutine I hope simple.
gin default use color on Term mode.
If user calls gin.DisableConsoleColor(), not use color.

[sairoutine]

@thinkerou

gin default use color on Term mode.
If user calls gin.DisableConsoleColor(), not use color.

This PR does not change this behavior.

If the user does not call gin.DisableConsoleColor() or gin.ForceConsoleColor(), gin decides to use color by Term mode.

If the user calls gin.DisableConsoleColor(), gin does not use color whatever isTerm=true or isTerm=false.

[thinkerou]

@sairoutine

This PR does not change this behavior.

Yes, I know. So I hope change it release gin1.4 before. do you think? thanks!

[sairoutine]

@thinkerou
Thanks! I also think this PR should be merged before gin 1.4 release.

[thinkerou]

@sairoutine I hope the pull request change to:

gin default use color on Term mode.
If user calls gin.DisableConsoleColor(), not use color.
can you recommit one patch? thanks!

[sairoutine]

@thinkerou
It seems that this PR is according to this. Could you tell me what should I do?

gin default use color on Term mode.
If user calls gin.DisableConsoleColor(), not use color.
can you recommit one patch? thanks!

[thinkerou]

@sairoutine OK, I see #1724 content about forceConsoleColor, you not need to do what. Thanks!

[sairoutine]

Thank your for your reviewing! Yes, This PR is related to #1724.