[ehmicky]

Fixes #620
Fixes #25

This allows the stdin/stdout/stderr to be an array of values instead of a single one.

Some interesting use cases: "taping" (mentioned in #25):

const { stdout } = await execa(..., { stdout: ['inherit', 'pipe'] })

Or redirecting to both terminal and file:

const { stdout } = await execa(..., { stdout: ['inherit', './path.txt'] })

The way this works is: we pass pipe to child_process.spawn(), and emulate the options ourselves through manual piping.
One caveat (which is documented by this PR): when using inherit + another value, even if the current stdin/stdout/stderr is an interactive TTY, the child process will not be interactive. This contrasts with using inherit on its own.