Automatically copy relevant text into user's clipboard

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.

Start by locating the implementation and output path for git node land --continue, especially where the Landed in ... instruction is printed. Determine how clipboard support can work on macOS, Linux, and Windows; done means the relevant text is copied for the user to paste directly into the pull request.