Support remote files API

[dleviminzi]

I would like to revive @ledmonster's effort to add remote file support.

[kanata2]

Thanks.

Please see this comment and update GoDoc.
#859 (comment)

[dleviminzi]

Thanks.

Please see this comment and update GoDoc. #859 (comment)

I made the change to the return parameters. I'm not sure what you mean when you ask me to update the GoDoc. Are some explanations not detailed enough or something else?

[kanata2]

@dleviminzi

Please add official API documentation's link 🙏

e.g.

slack/chat.go

Lines 185 to 189 in 03f86be

	// UnfurlMessageWithAuthURL sends an unfurl request containing an
	// authentication URL.
	// For more details see:
	// https://api.slack.com/reference/messaging/link-unfurling#authenticated_unfurls
	func (api *Client) UnfurlMessageWithAuthURL(channelID, timestamp string, userAuthURL string, options ...MsgOption) (string, string, string, error) {

[kanata2]

LGTM. Thanks!

[dleviminzi]

LGTM. Thanks!

Apologies, I forgot to update the example when I changed the return arguments.

[kanata2]

👍

[dleviminzi]

👍

Hello, when will this be able to get merged?

[kanata2]

merged!