5.x.x - Human readable size limits

Based on conversations in https://github.com/expressjs/multer/pull/848, we can change the limits to accept either a `Number` of bytes, or a human readable string like `100MB`.

We can use the [`bytes`](https://www.npmjs.com/package/bytes) module to achieve the conversion.

The docs and tests will need to be updated accordingly, to make sure users understand not only the change but the default behavior of using a Number of bytes.