Set user flair by flair_template_id

[Venefilyn]

There doesn't seem to be any support for setting user flair by flair_template_id. Only for managing flair templates.

SetUserFlairAsync() only supports text and cssClass

https://github.com/CrustyJew/RedditSharp/blob/d25ae76a4b29d3b7c36c0a050155111717dd0c17/RedditSharp/Things/Subreddit.cs#L665-L674