Add pipeStdout()-related methods

[ehmicky]

Fixes #525.

This adds the following shortcut methods: pipeStdout(), pipeStderr() and pipeAll().

For Node.js shell-like scripts, this is the equivalent to the |, |&, >, &>, or 2>&1 shell characters.

Pipe to stream, before:

const subprocess = $`echo foo`;
subprocess.stdout.pipe(process.stdout);
const {stdout} = await subprocess;

After:

const {stdout} = await $`echo foo`.pipeStdout(process.stdout);

Pipe to file, before:

const subprocess = $`echo foo`;
subprocess.stdout.pipe(fs.createWriteStream('stdout.txt'))
const {stdout} = await subprocess;

After:

const {stdout} = await $`echo foo`.pipeStdout('stdout.txt');

Pipe to another process, before:

const ffmpegProcess = $`ffmpeg -ss 5 -i input.mp4 -t 1 -vf cropdetect -f null -`
const awkProcess = $`awk ${'/crop/ {print $NF}'}`
const tailProcess = $`tail -1`

ffmpegProcess.stderr.pipe(awkProcess.stdin)
awkProcess.stdout.pipe(tailProcess.stdin)

const { stdout } = await tailProcess

After:

const {stdout} = await $`ffmpeg -ss 5 -i input.mp4 -t 1 -vf cropdetect -f null -`
  .pipeStderr($`awk ${'/crop/ {print $NF}'}`)
  .pipeStdout($`tail -1`)

The PR's code and tests are done, but I haven't added the types, type tests nor readme documentation. I'd like to first confirm the overall idea and design make sense to you @sindresorhus. :)

[sindresorhus]

This is very nice API sugar 👍

[ehmicky]

Those methods must be marked as optional since they depend on the value of the stdout, stderr, stdio, all and encoding options.

In the future, we might want to improve the types to infer the value of those options so they can be used to determine those methods' optionality. However, this would complicate the types quite a bit, so I'm leaving it off for this PR.

[ehmicky]

I have added types and documentation. The PR is ready to review 👍