Automatically copy relevant text into user's clipboard

先定位 git node land --continue 的實作與輸出路徑，尤其是列印 Landed in ... 指示的位置。確定如何在 macOS、Linux 和 Windows 上實作剪貼簿支援；當相關文字被複製後，使用者可以直接將其貼到 pull request 中時，即視為完成。

When running commands, eg:

$ git node land --continue
Running `final`..
  ✔  06e09b61712d4963699d90389b43bddcf41c02f0
     ✔  0:0      skipping fixes-url                        fixes-url
     ✔  0:0      blank line after title                    line-after-title
     ✔  0:0      line-lengths are valid                    line-length
     ✔  0:0      metadata is at end of message             metadata-end
     ✔  0:0      reviewers are valid                       reviewers
     ✔  0:0      valid subsystems                          subsystem
     ✔  0:0      Title is <= 50 columns.                   title-length
--------------------------------------------------------------------------------
The following commits are ready to be pushed to DANGER/master
- 06e09b6171 test: refactor http-https-default-ports
--------------------------------------------------------------------------------
To finish landing:
1. Run `git push DANGER master`
2. Post in the PR: `Landed in 06e09b6`
--------------------------------------------------------------------------------
? Clean up generated temporary files? [Y/n] Y

it should be possible to copy the Landed in ... to the clipboard, so the user can just change to browser and directly paste it in.

Not sure if possible on Windows, but on mac it's pbcopy, and on Linux it's normally xclip -selection clipboard.