* feat: consolidate ipfs.add input normalisation

Allows input normalisation function to be shared between ipfs and the
http client.

* feat: support pull streams (ipfs#8)

* feat: support pull streams

This PR updates the `normaliseInput` function to accept pull streams.

I've also made the following changes:

1. Update the docs for supported inputs
  * `Buffer|ArrayBuffer|TypedArray` is aliased as `Bytes`
  * `Blob|File` is aliased as `Bloby`
  * Added info for what a input "means" i.e. causes single/multiple files to be added
1. Peek the first item of an (async) iterator properly
1. Move file object check below `input[Symbol.asyncIterator]` check because Node.js streams have a path property that will false positive the `isFileObject` check
1. Fix `toFileObject` to allow objects with no `content` property
1. Simplify `toBuffer` to remove checks that `Buffer.from` already does

The service-worker-gateway was only responding with the correct content-type for some mime-types, see https://github.com/sindresorhus/file-type#supported-file-types.
This commit now fall-backs to detecting based on the extension as well.

Note that SVGs aren't supported by the `file-type` module.