Add options: auto-assign and assignees

[sorenlouv]

Closes #188

Example: assign yourself to the backport pull request

backport --auto-assign

if you always want to assign yourself to your backport PRs you can add the following to .backport/config.json:

{
  "autoAssign": true
}

Example: assign another user to the backport pull request:

backport --assign watson

[sorenlouv]

Released in v5.4.3