sdfmt: support for --offset and --length

[JohanEngelen]

clang-format supports only reformatting part of a file, by specifying --offset and --length pairs on the commandline (see https://clang.llvm.org/docs/ClangFormat.html). I use this in my editor by selecting a few lines, and then only reformatting the selected lines, rather than the whole file. Very useful for not combining code changes (with correct formatting) with formatting fixes in other parts of the file.

[deadalnix]

Just like #262 I don't have the bandwidth to make this happen but would assist. I think the right path forward here would be to leverage the skipFormat feature (triggered by sdfmt on/off style comments) to tell sdfmt to not format anythign outside of the the range. This is however likely to surface bug in that feature, which is not well tested at this time.