REST API: support Application emojis

[Big-Iron-Cheems]

As of now, discordgo's restapi.go supports control of Guild based emojis, but not Application based ones.
The docs related to Guild and Application emojis are located here: https://discord.com/developers/docs/resources/emoji.
I am interested in supporting all the endpoints that begin with /applications/.

Screenshot below is from the https://discord.com/developers/applications/APPLICATION_ID/emojis page.

Currently I have written this function in my personal project to GET a specific emoji by ID:

// applicationEmoji fetches an emoji from the application
func applicationEmoji(s *discordgo.Session, applicationID string, emojiID string, options ...discordgo.RequestOption) (emoji *discordgo.Emoji, err error) {
	var body []byte
	body, err = s.RequestWithBucketID("GET", discordgo.EndpointApplication(applicationID)+"/emojis/"+emojiID, nil, discordgo.EndpointApplication(applicationID), options...)
	if err != nil {
		return
	}

	err = discordgo.Unmarshal(body, &emoji)
	return
}

// Example usage in my project
emoji, err = applicationEmoji(s, applicationID, emojiID)

I am working on an implementation that is close to the Guild emoji functions.
Progress tracked at this PR, suggestions appreciated.