feat: enable stream picking on pipe()

[antongolub]

Closes #978

  const p = $`echo foo >&2; echo bar`
  const o1 = (await p.pipe.stderr`cat`).toString()
  const o2 = (await p.pipe.stdout`cat`).toString()
  
  assert.equal(o1, 'foo\n')
  assert.equal(o2, 'bar\n')

• Tests pass
• Appropriate changes to README are included in PR