Add ability to write to STDIN

[schneems]

• Add: Rundoc command background.stdin_write to send a string to a backtround process' STDIN. This allows driving REPL interfaces:

:::>- background.start("ruby #{script}",
  name: "script",
  wait: ">",
  timeout: 15
)
:::-- background.stdin_write("hello", name: "script", wait: "hello")
:::-- background.stdin_write("exit", name: "script", wait: "exit")
:::>> background.stop(name: "script")