Cross-platform newlines for --check

[bjones1]

I'd like to run cargo sort --check on Windows/Linux/OS X in CI. However, I get failures on Windows, since cargo sort wants to rewrite the line endings to be an LF, instead of CRLF. I don't expect setting crlf = true in tomlfmt.toml to help, since that will cause failures on Linxus and OS X.

Would you consider a crlf setting which selects CRLF on Windows and LF on Linux/OS X? I think that would solve this problem.

[jplatte]

On the surface, this feature request makes sense to me. I see there's some windows-specific logic in fmt.rs already, but haven't grokked the whole thing. If you could write up a small summary of what's currently there and what you plan to change, I could better gauge whether I'd be interested in a PR (this project is quite low priority to me, and so I haven't spent the time to fully understand what's here already since taking over maintenance).

In terms of UI, I think this could be expressed as crlf = "native".