Add --fix-dry-run option

[jopemachine]

Fixes #663.

Eslint doesn't expose fixDryRun option in programmatic API.
fixDryRun is internally translated to fix option and don't call ESLint.outputFixes.
(Ref: eslint/eslint#15877)

So I think xo could handle this option similarly.
fixDryRun is translated to fix option before passing to ESLint constructor, and xo.outputFixes would not be called since
the fix option is false.

[sindresorhus]

I feel like it's unclear to the user how this would even work. The user will ask themselves: "If it does not save to the file system, what's the point of it?"

[jopemachine]

--fix-dry-run     Automatically fix problems without saving the changes to the file system.
                  Because the default formatter does not output the fixed code, you'll have to use another one (e.g. json)
                  This flag can be useful for integrations which need to autofix text from command line without saving it to the filesystem.
                  Note that `fix-dry-run` is mutually exclusive with the `fix` option.

How about the above one?
I'm worried about only this option's description looks too verbose because the other options' descriptions are all single lines.
These descriptions are copy-pasted from https://eslint.org/docs/user-guide/command-line-interface#--fix-dry-run without the last sentense.

[mvllow]

Perhaps "Preview fixed file contents" or "Print fixed file contents"?

Suggested change

	--fix-dry-run Automatically fix problems without saving the changes to the file system
	--fix-dry-run Preview fixed file contents

[sindresorhus]

This does not make it clear how it then manages to be a "dry-run". The important part is missing.