Buffer output support

[florimondmanca]

Hi!

First, thanks for sharing this library. I'm opening this to discuss whether and how spawnAsync could support exposing the raw binary stdout content.

My use case is the following: I am using [pandoc] to convert HTML content to a binary format (.docx in my case). Pandoc outputs the raw docx content to stdout, but since spawnAsync concatenates the output in a string, the binary content is scrambled which makes the docx corrupted (probably because concatenating the binary output to a string force-converts to utf8).

For this reason I couldn't use spawnAsync and had to use the built-in spawn function and basically reimplement error handling and reading from the process stdout stream.

Of course we don't want this change to break backwards compatibility, so SpawnResult.stdout should still contain a string.

What I'd imagine is a new attribute on SpawnResult containing the raw Buffer[] content, e.g.:

process = spawnAsync('pandoc', ['--from=html', '--to=docx']);
process.child.stdin.end("<h1>Hello, world</h1>");
result = await process;
const docxContent = Buffer.concat(result.raw); // <--

I'd be happy to work on this, if this sounds like a reasonable feature to add!

[ide]

This sounds reasonable. I like the idea of adding new properties rather than doing what Node does with spawnSync where stdout/stderr can be either strings or Buffers based on the encoding option. Could you add rawStdout and rawStderr along with tests for both?