Release 2.0

This is a tracking issue for release 2.0.

I am trying to give better visibility for upcoming changes and so am trying out making a PR for a release here, pulling from the official next release branch into `master`. This allows for the current pending changes to be easily visible.

**Please keep feature requests in their own issues**

I'm also leaving this PR unlocked so people can make comments/etc. and we'll see how it goes :) If you want to make a comment on a particular change, please make the comment in the "Files changed" tab so comments are not lost during a rebase.

List of changes for release:
- [x] `req.body` no longer always set to `{}`, which works better for non-object-like bodies like text
- [x] `on-finished` used to detect if the request has been read; no longer need to set `req.body` to a truthy value
- [x] `urlencoded` no longer defaults `extended` to `true`

List of deprecated things removed:
- [ ] Main `bodyParser()` function which did `urlencoded` + `json`

**Testing this release**

If you want to try out this release, you can install it with the following command:
```bash
$ npm cache clean
$ npm install expressjs/body-parser#2.x
$ npm cache clean
```