add support for specifying Faraday Middleware

[markburns]

@bernat might be the best for reviewing this.

I'm not completely happy with this approach as it feels like I had to touch too many places, but functionality-wise it serves the purpose.

This allows us to do

middleware_stack = [[Faraday::HttpCache, {cache: Rails.cache}]]

Svelte.create(json: json, module_name: module_name, 
  options: {middleware_stack: middleware_stack}
)

[markburns]

Hey all. Just going through and tidying up my old PRs. I'm guessing you haven't needed this so going to close.