Add backport label on repos where it exists

[epixa]

It's really useful for glancing at a list of PRs or auditing PRs to be able to filter away any PR that has a backport label, so it'd be great if this tool added that label automatically to all backport PRs.

[sorenlouv]

I tried doing this, but Github's API didn't seem to let me add labels to PRs. Will make this a priority now.

[epixa]

You may want to take a look at the elastic-jasper github code to see how I did it there. I can't remember off-hand

[sorenlouv]

@epixa I've created a PR #12 that allows the user to labels to their config file, which will be added to the backport PR. Like this:

{
  "name": "elastic/kibana",
  "versions": ["6.x", "6.0", "5.6", "5.5", "5.4"],
  "labels": ["backport"]
}

I also considered making it a handler (although that seems unnecessary for now)

{
  "name": "elastic/kibana",
  "versions": ["6.x", "6.0", "5.6", "5.5", "5.4"],
  "labels": version => something ? ["backport", version] : [version];

}

What do you think?