Support multipart/form-data operations

[erunion]

I made a bunch of changes in readmeio/httpsnippet#1 to allow for Node request snippets with params that have fileName to surface that instead of the data URL, we should copy those changes over here into our code snippet for the SDK.

[erunion]

Snippets should look like:

sdk.post('/anything', {
  documentFile: 'owlbert.png',
})
  .then(res => res.json())
  .then(res => {
    console.log(res);
  });

The SDK should handle creating read streams for fetch instead of offloading that work to the user, but for the sake of magic we should support passing streams too.

[erunion]

fetch-har@4.0.0 has support for multipart/form-data so this should pull that in.