Execute requests asynchronously and support aborting them

[dkvasnicka]

I forgot to put the proper port to my hostname, executed the request and the whole Vim instance froze, probably because cURL was waiting for a response from a server that did not have port 80 open.

Could you make it so that the requests are executed using the async capabilities of Vim 8 / NeoVim and allow the user to abort a request?