Disable formatting of code blocks by default

[abhinav]

Creating this with regard to this comment originally posted by @karelbilek in https://github.com/Kunde21/markdownfmt/issues/38#issuecomment-1277758754:

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To quickly explain - the original usage for this was mainly for go samples in code documentation.

I think the original behaviour doesn't need to be enabled by default... I think default behaviour should be to just not do any reformatting.

(I am surprised that Uber is using our stuff, but also happy! :D )

Btw maybe some added tests would be nice...

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So this issue is to discuss/track whether the auto-formatting of code blocks should be disabled by default.

My opinion: I agree with @karelbilek. By default code blocks should be left alone, reproduced exactly as-is in the final output.