Implement support for async requests

[Malabarba]

I did this mostly because request.el is bugged for me on synchronous requests. But I think it's a nice addition anyway (it has bothered me many times before that emacs would hang while the request is running, specially when I setup some debugging breakpoints in the server).

Hope you find this useful.

Thanks a lot for this package.

[Malabarba]

Changing the parser is an independent change. Instead of calling json-read here only to call json-encode later, we can just take the buffer contents and no do any encoding/decoding.

[davazp]

Nice! That is a nice improvement.

I think I'd like a small change.

I would like to keep a function that is useful to make requests programatically. Before this change, that function was graphql--query and graphql-post-request. But now the callback in them means you can't build your own functions on top of it.

I'd propose to add two arguments sync and success callback arguments to them, and only pass the callback from the interactive graphql-send-query function.

Then it is probably a good idea to still keep the :parser 'json-read. But feel free to add still another argument to it and pass buffer-string from the query if you want.

Hm, with those changes, I also feel comfortable of making the interactive function always async and not even introduce a custom.

Thoughts?