[WIP] Auto-assign labels to PRs based on their title.

Testing with different repos:

```bash
$ export GH_TOKEN=... # valid token unfortunately required if you want to test without modifying the source
$ NEOVIM_REPO=fwalch/neovim SIMULATE=true ./ci/assign-labels.sh
Local build, skip installing dependencies.
Processing only PRs.
Updating [RDY] issue 5.
  Adding 'ready' label.
  Removing 'in progress' label.
$ NEOVIM_REPO=fwalch/neovim SIMULATE=true PROCESS_ONLY_PRS=false ./ci/assign-labels.sh
Local build, skip installing dependencies.
Updating [RDY] issue 5.
  Adding 'ready' label.
  Removing 'in progress' label.
Updating [RDY] issue 4.
  Adding 'ready' label.
$ NEOVIM_REPO=neovim/neovim SIMULATE=true ./ci/assign-labels.sh
Local build, skip installing dependencies.
Processing only PRs.
Updating [RFC] issue 1414.
  Adding 'in progress' label.
Updating [RFC] issue 1411.
  Adding 'in progress' label.
Updating [RFC] issue 1403.
  Adding 'in progress' label.
Updating [RFC] issue 1399.
  Adding 'in progress' label.
Updating [RFC] issue 1391.
  Adding 'in progress' label.
Updating [RFC] issue 1389.
  Adding 'in progress' label.
Updating [RFC] issue 1383.
  Adding 'in progress' label.
Updating [RFC] issue 1373.
  Adding 'in progress' label.
```

Only thing left to decide is whether all issues or just PRs should be processed, then I'll probably remove that option. I thought about splitting the code into multiple functions to improve readability, but didn't do so for now.

I didn't find a way to get more than 30 issues from the Github API.

Ping @elmart.
Repeating "in progress" loses information.
I would suggest some other possibilities for labels:
- in-progress / ready-to-review / ready-to-merge
- in-progress / in-review / ready
- in-progress / reviewable / mergeable
- working / reviewing / ready
- WIP / RFC / RDY