feat(scripts) introduce merge-master-into-next script
feat(scripts) introduce merge-master-into-next script

This is a development tool for performing merges from the `master` branch into
`next` correctly. Up to this day, these merges had to be done manually and
that process was error-prone (multiple people got it wrong in different
occasions, myself included).

A key problem is that the Github green button offers three options but none of
them does the right thing we need (you end up either with an incorrect rebase,
or with a double merge commit), therefore the branch needs to be approved in
the web UI and merged from the terminal.

This script makes this process more consistent. It requires `hub` (a CLI
interface to the Github API).

As a demonstration of this script, the latest merge from `master` into `next`
was done entirely using this script:

* I ran `scripts/merge-master-into-next create` and it created the branch locally
* Then `scripts/merge-master-into-next submit` and it automatically opened
PR #4741 (including the "do not merge" label, etc.)
* @gszr approved it in the web UI
* Then I ran `scripts/merge-master-into-next merge`, the script merged the branch,
closed the PR and deleted the repo in Github.

Kong/kong,Mashape/kong,Kong/kong,Kong/kong