Execute external program (capture)

---

examples/process/capture2.cr

output, error, exit_code = capture("ls", ["-l", "-a"])
if exit_code == 0
  puts output
else
  puts error
end
puts exit_code

def capture(cmd, params)
  stdout = IO::Memory.new
  stderr = IO::Memory.new
  res = Process.new(cmd, params, output: stdout, error: stderr).wait

  return stdout.to_s, stderr.to_s, res.exit_status
end