Always skip children when using Plain write mode

[MicahChalmer]

Outputting child module contents in the "Plain" write mode does not make sense, since there is no way to differentiate code that came from a child module from that which came from the parent file.

Fixes #939. If this is merged, integrations that want to run a single file can send it on stdin and need not specify any flags--the output will contain only the reformatted code that was input.

[nrc]

Looks good, thanks!

[lhecker]

@nrc would it be possible for you to release a v0.4.2 or something? I modified the very widely used atom-beautify extension today to use this new change and it works not just faster but als much better, but it depends on this PR. For instance it finally fixes the problem that it didn't work on lib.rs and mod.rs files because it couldn't find the files behind the mod foobar; declarations. So pushing a new version right now might possibly really benefit a lot of Rust users. 😊

[nrc]

@lhecker I'm planning on doing a 0.5 release soon, just waiting for a PR to land, should happen in the next few days. Is that soon enough?

[lhecker]

Yeah of course! I'll submit a PR for a rustfmt 0.5 to atom-beautify and let it merge as soon as you published the new version.