Add Attachment Upload API method

[baudev]

Add the method getAttachmentUploadId() which allows sending files with external urls. It uses the Facebook Attachment Upload API.

You can use it as below:

bot.getAttachmentUploadId("https://images.pexels.com/photos/63324/california-road-highway-mountains-63324.jpeg?auto=compress&cs=tinysrgb&dpr=2&h=750&w=1260", true, "image", (err, body) => {
    // ...
});