sdfmt: support stdin and stdout, together with --assume-filename

[JohanEngelen]

The feature request is to be able to input code via stdin, and then outputting to stdout.
Then for the style to be applied, one should be able to specify --assume-filename (by which then the normal config file lookup would be used).
Clang-format also has this --assume-filename option (https://clang.llvm.org/docs/ClangFormat.html).

[deadalnix]

Sorry I missed this. I'd don't have the bandwidth to make this happen right now, but I'd be happy to assist and/or review any work done on that front. This seems like a good feature.

[JohanEngelen]

response is appreciated, notifying contributors (e.g. me) that the work will not be in vain

[deadalnix]

Definitively not, being able to format from stdin is a very useful feature. maybe even be the default if no file is specified on the command line?

I'd be happy to provide pointer through the codebase to do that, but for the most part this should probably happen in the driver for sdfmt or close by.